The classic manual wind chronometre in the larger 35.5mm case but with a refinished dial. It has the famed 30mm chronometre movement with the Rg regulator. About 20,000 of the these movements were made and were housed in either a 33mm or 35.5mm snapback case or the uber rare 35mm screwback case. Many, many of these watches had refinished dials and not many survive with original dials. These were the movements that put omega on the map as they won several chronometre competitions beating similar models from patek, vacheron, rolex etc. The other big winner in these contests was zenith the the harder to locate cal. 135 manual wind chronometre movement.