It had a case measuring 40mm in diameter. The movement also changed a bit and became Rolex caliber 1575. Later, the GMT-Master 1675 evolved with a caliber 1570 inside, the base caliber for this self-winding movement is 1570 and was left engraved on the bridge of caliber 1575. There also have been production numbers of the 1675 GMT-Master with caliber 1565 but it is unclear when these were used and in which particular models. The 1675 GMT-Master was in production for a long time, from 1959 till approximately 1980. Here is one from the 70s.