[drnosker]

All modern Rolexes have a free-sprung balance (perhaps with the exception of the Cellini line.) This means that regulation is not by the more common method of adjusting the effective balance spring length with curb pins mounted on a regulator arm, but by adjusting the weights mounted in the plane of the balance itself. This method can provide much finer regulation, and was in fact used by marine chronometers before use in watches. By the way, Rolex began the use of this method first on a limited basis on the KEW observatory speedkings of the 1940's, and then in regular production on 1065 and 1066 models in the 1950's. The last oyster model to use the old method of regulation would be the no date 5513 submariner of 1989 or 1990.