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 normally engraved  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 is seen in the 1675 until the 1.4m serial range.
In 1965, Rolex rolled out the 1575 movement, which beat at a faster 19,800 VPH and was engraved  for its base calibre. Gilt dials and the transitional Mark 0 matte dials with serials earlier than 1.4m are usually seen with the 1565 movement and those after usually have a 1575. Like most transitions, there are correct watches on either side that ‘break’ this rule of thumb.
The 1575 movement was later upgraded with a hacking function around 1971; this version of the 1575 remained the same throughout the rest of the 1675s production run.
Picture Credit: S. Song Watches