In the Beginning:
Here is an excerpt from the Dowling-Hess book;
Best of Time, considered to be one of the best researched and authoritative books on Rolex.
"Travelers loved the fact that trans-Atlantic journey times were now halved, from 13 hours to seven, but they suffered from a new phenomenon: "Jet Lag." Simply put, the body's internal rhythms were knocked off balance by the sudden difference between time zones 5 or 6 hours apart. Pan-American [arlines] was worried enough about the effect on its pilots to commission research, and the result was that they were advised to keep their pilots on "home" time during the period they were away from base. At the same time, they needed to be aware of the local time at the various venues.
To meet both of these needs Pan Am asked Rolex to develop a watch. A joint task force from both companies was assigned the problem. Pan Am's team was lead (sic) by the inimitable Captain Frederick Libby, one of the original "Skygods" (the name other Pan Am pilots gave to the original cadre of pilots who had flown for Pan Am before World War II), while the Rolex team was led by the redoubtable Rene'-P. Jeanneret. Jeanneret was called the company's Head of Public Relations, but in fact was the company's Jack-of-All-Trades (and master of all of them). In remarkably short order the task force produced the "G.M.T. Master" (model no. 6542), named after Greenwich Mean Time, the world's standard time, the GMT-Master was a heavily revised version of the "Turn-O-Graph" (model 6202)... "
Thankfully the watch became a hit and is still with us today in the form of the GMT II, although the original GMT Master was available from 1954 until discontinued in ~1999, a remarkable 45 year run...
The GMT is unique in that there was almost always more than one GMT to choose from and manufacturing of various models overlapped by quite a bit..
Here is a timeline chart of the different models and when they were available. This chart is from
www.gmtmasterhistory.com , an excellent site with much information available..
timeline.jpg
Links:
Early GMT Development History
The Original plastic insert mystery
The GMT-Master
GMT references
Evolution of the GMT--Part I
Evolution of the GMT--Part II
16713, Y serial
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Pepsi inserts.jpg
GMT insert information courtesy of Jocke
You ask to identify the GMT bezel insert numbers>>>
1675 and 16750 with plastic crystal use the same inserts.
Sapphire models use the same inserts.
The part numbers are the first 3 digits means it´s a bezel insert. The next 4 or 5 is
for the reference number of the watch and the last number is for the color.
So here we go.
Black with white numbers 315-16750-1
Red/Blue with white numbers 315-16750-6
Bronze/Yellow with yellow numbers 315-16753-5
Black with yellow numbers 315-16753-1
Black with yellow numbers 315-16758-1
Bronze with yellow numbers 315-1675-3
Bronze with yellow numbers 315-16758-3
Black/Red with white numbers 315-16760-7
Black with white numbers 315-16700-1
Red/Blue with white numbers 315-16700-6
Bronze/Yellow with yellow numbers 315-16713-5
Black with yellow numbers 315-16718-1
Bronze with yellow numbers 315-16718-3
Insert sizing:
The 1675/16750 ...............have an I.D. of 30.2 and an O.D. of 37.75mm
The 16700/16760/16710.... have an I.D. of 30.7 and an O.D. of 37.65mm
The Rolex GMT Master
©Write Time Partners V, 1999
Rene-Paul Jeanneret was one of the most important executives at Rolex Geneva in the company's most fruitful period, the 1950s and 60s. His official title was that of Public Relations Director; but he was so much more than that, he involved himself in many aspects of the company's activities including inventing watches; although his name appears on none of the Rolex patents from that period. An active sportsman in many fields including skiing and the newly introduced skin diving; in the early 1950s he came up with the concept of "tool" watches. This was the idea of a watch specifically designed for practitioners of an individual sport or activity. The results of this concept appeared on the Rolex stand at the 1954 Basle Watch Fair in three forms; the Explorer for sportsmen, the Submariner for divers and the Turn-O-Graph for businessmen. The watches all proved to be great successes and so it was no surprise when the world's largest airline Pan-Am wanted a watch that would enable their pilots to keep track of time in two locations; it was to Jeanneret that they turned.
Working in conjunction with Pan-Am.'s Captain Frederick Libby (a decorated World War II veteran and one of the airlines most respected navigators), Jeanneret came up with the idea of a watch with an additional hour hand revolving just once every 24 hours and a rotatable bezel marked with those same 24 hours. The watch itself was a typical Rolex product, it was simply a regular 6202 "Turn-O-Graph" with a different bezel and the 1030 movement normally fitted to Turn-O-Graphs had an additional 24 hour driving wheel and a calendar disk; this, and the fact that the movement was now chronometer certified, allowed Rolex to give the movement a new reference number, 1065. The external look of the watch was very similar to the contemporary Turn-O-Graph and Submariner; it was still quite a slim watch and without the crown protecting "shoulders", looked considerably smaller than the current model. The GMT Master was also important in that it was one of the first Rolex model to feature the new "Cyclops" lens from the introduction of the watch. These first GMT models (ref. 6542) are immediately recognised by the bright plastic bezel insert. This plastic bezel insert was the first item to be changed in 1956, giving way to a metal insert with the numbers now screen-printed. These new bezels were less likely to crack than the earlier plastic ones but were much more likely to fade in bright sunlight.
Due to Pan-Am.'s ever increasing fleet of Boeing 707s the vast majority of the early watches produced went to the company, each plane had a Pilot, a First Officer and a Navigator all of whom were issued with GMT Masters and Pan-Am had several hundred crews all issued with a company GMT Master. These watches bore no company logo; no "Property of" markings and no special dials, apart from one strange bunch of 100+ watches made in 1958 especially to solve a problem inside the airline's Chrysler Building head office (they did not move to the new Pan-Am building until the early 1960s). The problem was simply that as the watches arrived in the head office before being sent off to the field offices for issue to the flight crew, they would be requisitioned by senior management who felt that they, rather than the flight crews, were the ones who deserved a new company Rolex. This happened on a regular basis until one day Juan Trippe, the mercurial head of Pan-Am glimpsed one of the watches on the wrist of an executive and wanted to know why it was not on the wrist of a pilot. The situation was explained to him; the pilots had everything, the gold braid, the titles, and the brand new Boeing jets and now they even got great watches. The executives felt shunned, they saw themselves as the basis of the company's success but were fed up of being treated as second class citizens. Trippe did not like the situation and ordered that all the GMT Masters in the building should be returned to the operation department for subsequent issue to flight crews. However to mollify the executives Trippe had Rolex manufacture a batch of 100+ GMT Masters solely for the "desk pilots"; these differed from the flight crew (and all other) GMT Masters in that they had white dials. They are believed to be the only GMTs made with this colour dial, the order proved a godsend to Rolex as they made these watches in 1959 with the last of the old model 6542 cases; for the new model was waiting in the wings.
The new model (1675) proved to be such a successful model that, unlike the Submariner, it continued almost unchanged for around 25 years. It now featured crown protecting "shoulders", silver printing on the dial, rather than the gilt colour used on the 6542, and the dial itself was now inscribed "Superlative Chronometer Officially Certified", rather than just "Officially Certified Chronometer" as on the 6542. The new legend came about because the watch was now powered by the new 1565 calibre which featured the free sprung "Micro Stella" balance; this new base calibre (1530) was capable of much greater accuracy than the earlier movements and was subsequently fitted into all Rolex models.
Although the watch had been made to a Pan-Am requirement many other pilots came to depend on the watch; particularly military pilots who kept the rotating bezel at “12”, thereby enabling the watch to give them both civilian and military (or 24 hour) time. Amongst the pilots who came to rely on the GMT Masters were the cadre of probably the finest (certainly the bravest) pilots in the world; those chosen by NASA to fly the North American X 15. The history of the X-15 deserves more than a throwaway paragraph; so I have taken the liberty of quoting from NASA's official history below.
An unofficial motto of flight research of the 1940s and 1950s was "higher and faster." By the late 1950s the last frontier of that goal was hypersonic flight (Mach 5+) to the edge of space. It would require a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge. It was designed to fly at speeds up to Mach 6, and altitudes up to 250,000 ft. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 ft. Looking at it another way, Mach 6 is about one mile per second, and flight above 265,000 ft. qualifies an Air Force pilot for astronaut wings. The plane was air launched by NASA's converted B-52 at 45,000 feet and a speed of 500 mph. Generally there were two types of flight profiles: high-speed, or high-altitude. High-speed flights were usually done below an altitude of 100,000 feet and flown as a conventional aeroplane using aerodynamic controls. High-altitude flights began with a steep, full-power climb to leave the atmosphere, followed by up to two minutes of "coasting up" to the peak altitude after the engine was shut down. "Weightless" flight would last for 2 - 5 minutes as it made a ballistic arc before re-entering the atmosphere. A reaction control system was used to maintain attitude above the atmosphere. The reaction controls employed hydrogen peroxide thrusters located on the nose and wings. A typical research flight lasted about 10 or 11 minutes while covering nearly 400 miles along a course that stretched from Smith Ranch, Nevada to Edwards Air Force Base. The X-15 program made many accomplishments; here is list of some of its contributions to space flight:
First use of a full-pressure suit for spaceflight.
First use of reaction controls for manoeuvring in space.
First use of a flight control system that automatically blended aerodynamic and reaction controls.
Development of thermal protection for hypersonic re-entry.
Development of the first large, restartable, and throttleable rocket engine.
Development of an inertial guidance system.
Demonstration of a pilot's ability to operate in "micro-gravity".
Demonstration of the first piloted re-entry-to-landing from space.
Acquisition of hypersonic acoustic measurements, which influenced structural design criteria for Mercury capsule.
Verification of the validity of hypersonic wind tunnel data, which were later used in the design of the Space Shuttle.
One of those pilots, Pete Knight, wrote the following to Rolex after one flight. “I finally flew on October 3, 1967 to a speed of 4,534 mph (7,269 kph) or Mach 6.72 and all systems functioned properly with the exception of some local heating damage on the lower ventral. I have been wearing my Rolex GMT-Master for a period of months now and have calibrated it to within a few seconds a day” The throwaway phrases “Mach 6.72” and “some local heating damage on the lower ventral” disguised the real facts. Knight had flown the X-15 to a world record speed that still stands over 30 years later and had brought the plane home after temperatures later determined to have been above 1650°C (3000°F) burned a ramjet engine off its pylon and seared a hole measuring 18 by 8 centimetres into the ventral fin's leading edge. An airscoop effect channelled hot air into the lower fuselage and damaged the propellant jettison system meaning that Knight eventually had to land the plane 680 kilograms heavier than planned because he could not jettison the residual fuel. If the heat had damaged the craft's hydraulics, Knight might have had to abandon the plane. Fortunately, that did not happen. Knight landed at Edwards Air Force Base with the plane resembling burnt firewood. It seems amazing that the plane made it and even more so that despite acceleration of more than 3.5G the Rolex GMT Master on his wrist also performed perfectly. The other X-15 pilots also wrote to Rolex about their watches but the story of Pete Knight’s final flight is perhaps the most interesting.
It is worth noting that while Knight wore a Rolex GMT Master on his world record flight he was following in the grand tradition of Edwards Air Force Base, for it was there in 1947 that Chuck Yeager first broke the “Sound Barrier” in a Bell X-1 and did so wearing a Rolex Oyster; the same one he had worn all through World War II.
The company perceived the GMT almost solely as a pilot’ watch the 1960’s brochure for the watch even included the following instructions for pilots in how to use the watch as an emergency compass. "In the Northern Hemisphere the Rolex GMT-Master may also be used as a compass. Simply point the hour hand towards the sun and automatically the red 24-hour hand will point to the North! Try it...! In the Southern Hemisphere it will point to the South." Whilst it had been the introduction of the Boeing 707 that had caused Pan Am to commission the GMT Master, it was the people who flew on them as passengers who became the main customers for the watch. With the introduction of jet travel, many people were now travelling between countries and of course between time zones. For these new international travellers the GMT Master was the answer to their prayers; however it was the use by these people which was the impetus for the next changes in the GMT Master. The fact was that anyone rich enough to travel by aeroplane in the 1960s was rich enough to afford a gold (not a steel) watch; and so in answer to this demand Rolex began to make the GMT Master in both 18k gold and in two tone. Interestingly the gold version came first and used the (by now) retired model number 6442 and was "shoulderless" whilst the new two tone version retained the same model number as before (1675) but both were distinguished by their new brown coloured dials and bezels; these watches first hit the market in late 1963. A little known fact is that these brown dial GMT Masters were the very first Rolex watches to have the luminous indices inside gold circles; a feature now seen on all Rolex sport watches. Probably as little known is the fact that the metal circles on the dials of steel watches are actually white gold, not steel.
As I said before; the concept of the GMT Master was so perfect that there was almost no need for different versions and models; the word “almost” is used because there was one variation of the GMT Master, but one that no-one sees as a GMT Master; that variation is the Explorer II. The only difference between the GMT Master and the Explorer II is that the Explorer II has a fixed bezel, whilst the GMT Master has a revolving one; the Explorer II is the only other Rolex model to use the calibre 1575 movement. Because the Explorer II offered only civilian and military time, rather than two time zones, it proved much less useful to customers and so never sold as widely as the GMT Master. Ironically, it is this relative unpopularity that has made the Explorer II one of the rarest and collectable of all 1970’s Rolex models. The design philosophy behind Rolex was always one of gradual improvement of an existing product, rather than changing the complete watch, as many other makers would do. We see this process perfectly when we examine the small, incremental changes to the GMT Master in the 70’s and 80’s. Firstly from around 1976 the “hack” seconds feature was added and seven or eight years later the “quick-set” date feature arrived. Both of these features are interesting in that they were designed to make the watch easier to use and also that both of them increase the perceived accuracy of the watch; because if it is easy to synchronise the watch to a known time signal, then it is just as easy to compare it to any subsequent time signal. Similarly if the hands do not have to be moved to change the date on months without 31 days then the accuracy of the watch can be maintained without re-setting the time.
At the end of the 1980s the 1675 was replaced by two new models; the direct replacement was the model 16700 GMT Master which was the first steel GMT Master to be available with white gold circular settings for its luminous hour markers. The new introduction was the [16760]16710 GMT Master II, this watch used the new calibre 3085 movement which allowed the hour hand to be moved forward or backwards in precise one hour jumps, this feat could be performed without losing the precise accuracy which was usually the reason the watch was bought in the first place. The new GMT Master II was available in all metal combinations, whilst the 16700 GMT Master was only available in steel. Despite the introduction of many watches by various other manufacturers; all of which claim to be the aviator’s watch, the reality is that the GMT Master remains the ultimate timepiece for pilots and all those whose lifestyle requires knowledge of multiple time zones, no matter whether it is for international telephone calls or intercontinental travel.