I don't really know with any authority, but I can hazard a guess. I believe the magazines are actually interchangeable. The MkII has a bolt hold-open on the last shot, but that's probably a modification to the bolt to catch the follower, not the other way around. I have a MkI and I love it. It was my first handgun and once I figured out how to use the safety to hold the slide open when I wanted to, I didn't feel handicapped at all.