It's possible the quartz revolution put pressure on demand and production of the Speedmaster Professional in the '70s and '80s, but they're around.
There aren't many changes to distinguish those watches from one another, or from a brand-new one, for that matter. The changes to the Speedmaster really occurred in the '60s, when the crown-guard was added to the case (1965-ish, I think), and then when the movement was changed from caliber 321 to caliber 861 (1968).
The difference in the two calibers is in the chronograph control, with c.321 using a classic column-wheel arrangement and c.861 using a simpler cam-activated arrangement. Caliber 1861 in a brand-new Speedmaster Pro today is the modern evolution of caliber 861 (one may say they're the same movement, basically).
They're all a joy to own and wear. The level of character and charm is remarkable, with a fascinating blend of shapes, materials and finishes all adding to the watch's character. Proportions are just right for modern trends. Legibility is fantastic. Winding it each morning is a pleasure. I'm pretty sure NASA selected it because the Astronauts thought it was the coolest one of the bunch they tested!
I used to wear a '67 reference 145.012, which was the last model to use c.321, but I sold it a few years ago to fund another purchase. Fortunately for me, I had a milestone birthday a couple of years ago, and my wife surprised me with a new Speedmaster Pro. I enjoy it immensely, and I know you will, too, when you find the right one for you.
IMG_3735 by
Tony Cariffe, on Flickr
IMG_1493 by
Tony Cariffe, on Flickr