I believe the Z serial models are older like V2 or older. The Z serial ones had black painted rehaut "engravings" too where the V serial engravings are not colored. I've owned the GMTIIC 5 or 6 different times since it came out and the latest one I picked up is a V3 with V serial.
V3 upgrades are a thicker bezel assembly which results in a slight recess of the ceramic bezel and a more proper crystal height. The bezel numbers are brighter, larger, and deeper cut. The crown is slight shorter.