Here are some features of the MARK I, II, and III pistols (changes from the previous version are highlighted in blue):
Pre MARK I/Standard:
Drift adjustable sights and tapered barrel.
No last shot bolt hold open.
Bolt can be manually locked back using the safety.
Heel mounted magazine latch.
9 round magazine.
MARK I:
Heavy tapered or bull barrel and target sights.
Trigger has adjustable stops.
No last shot bolt hold open.
Bolt can be manually locked back using the safety.
Heel mounted magazine latch.
9 round magazine.
There are also "old and "new" model MKIs (and standards). The A100 frame was introduced in 1971 and the magazine follower button moved to left side.
MARKII:
Standard has drift adjustable sights and tapered barrel.
Target Models have heavy tapered or bull barrel and target sights (also the trigger has adjustable stops).
Trigger pin retention method changed.
Last shot bolt hold open via added bolt stop.
Safety no longer used to lock bolt back.
Heel mounted magazine latch.
New 10 round magazine.
MARK III:
Standard has drift adjustable sights and tapered barrel.
Target Models have heavy tapered or bull barrel and target sights (also the trigger has adjustable stops). There is also a Hunter version with fluted bull barrel.
All Mark III pistols with adjustable sights are drilled and tapped for a Weaver-style scope base.
Last shot bolt hold open via bolt stop
Loaded chamber indicator added.
Magazine disconnector added.
Internal lock added.
Magazine latch moved to left side of the frame behind trigger guard.
Moved latch requires different style 10 round magazine.
There are also some differences with the early pre-MK1 pistols (there was no Standard moniker until the MK I was introduced) and latter production ones.
Rifling twist rates were changed during the production of the MARK II to the current 1 in 16".
There are also some differences in the bolt ears on MK II and III pistols.
I also believe there have been some bull barrels made with drift adjustable sights fitted.
22/45:
They incorporate a different grip angle, polymer frame with the magazine latch located on the left side behind the trigger guard (both version 22/45s have a magazine base plate different from the steel frame versions).
The original 22/45 upper is very similar to the MARK II; having different markings and possibly differences in the lug area.
The new 22/45 has a redesigned frame (less bulky than the earlier versions) and incorporates the MARK III magazine disconnect and internal lock. The upper also has the MARK III loaded chamber indicator.
Some parts will interchange.
Accuracy is about the same for all the MARKs; varying more pistol to pistol rather than MARK series. It would take a lot of shooting to degrade the accuracy of these pistols, so old ones should be fine (abuse aside).
I like the last shot hold open, but dislike the loaded chamber indicator and magazine safety; so I prefer the Mark II version. I also like the MARK I and have one made in 1953 that is a great shooter. I have no love for the MARK III.
Regards,
Greg