Yes, the M2 is the Mark II. It is the predecessor of the M3 or Mark III. I believe the difference is with the size of the hole. Older Hulme case feeders did not have the T slot, so they couldn't be used to load 44 Magnum, 45 Colt or 45 AR. Correspondingly, the hole in the Top of the M2 Top Plate was not big enough to handle the rim on the cartridges. With the M3 and a Hulme case feeder that has the T Slot, you can load 44 Special and 45 AR. Of course need an M3A Top Plate to load 357 Magnum, 41 Magnum, 44 Magnum, and 45 Colt. You do not need a case feeder with the T Slot to load 41 Special or 41 Magnum.
Note: Many M2 top plates have been modified so they will handle the larger calibers.
Bruce Williams