The early Explorer 1016 came with what was, at the time, a new “microstella” regulated 1560 calibre movement. These movements were the first for Rolex to certify as “Superlative Chronometer Officially Certified” (SCOC) in the Explorer line and receive this designation. However, the first dials can still be seen with the older “Officially Certified Chronometer” (OCC) text as they had not updated the dials from the older 1030 and 1060 calibre designations.
Here is an article discussing the larger history of Rolex’s chronometer certifications, which I will not touch on here. The movements numbers are engraved on the automatic winding bridge. The 1560 movement, which is seen on Explorer in the early 1960’s, beat at 18,000 VPH and were made until around 1967. As early as 1965 Rolex rolled out the 1570 movement, which beat at a faster 19,800 VPH. The 1570 movement was later upgraded with a hacking function sometime in 1971; this movement then remained the same throughout the rest of the 1016 production run.