The early GMT-Master 1675 came with what was, at the time, a new “microstella” regulated 1565 calibre movement. These movements were the first for Rolex to certify as “Superlative Chronometre Officially Certified” (SCOC) in the GMT-Master line, however the first 1675 dials can still be seen with the older “Officially Certified Chronometer” (OCC) text as they continued to use the older 1030 and 1060 calibres.
Here is an article discussing the larger history of Rolex’s chronometer certifications, which I will not touch on here. The 1565 movements are often engraved 1560 on the automatic winding bridge, which is the base calibre certified movement on which the GMT and date functions were added. The 1565 movement, which is seen on GMT-Masters as early as 1960, beat at 18,000 VPH and was made until around 1967. As early as 1965 Rolex rolled out the 1575 movement that beat at a faster 19,800 VPH and was engraved 1570 for its base calibre. The transitional Mark 0 matte dials can be seen with either the 1565 or 1575 movement. The 1575 movement was later upgraded with a hacking function sometime in 1971; this movement then remained the same throughout the rest of the 1675 production run.
Picture Credit: S. Song Watches