Over time, I have owned all three. But it's been over ten years since I've owned a Mk I, and now I really can't remember what set the Mk I apart from the Mk II.
I believe the Mk I didn't have the bolt hold-open device, and there was a slight difference in the magazines; the MkI mag could be modified to fit a Mk II, but not vice-versa. Again, the Mk I/II/III experts are at markii.org.
I believe the Mk I didn't have the bolt hold-open device, and there was a slight difference in the magazines; the MkI mag could be modified to fit a Mk II, but not vice-versa. Again, the Mk I/II/III experts are at markii.org.