I have them all -- the Mark I, II, III, and IV. All with 5 1/2" bull barrels, and all of which I bought new.
All but the Mark I needed serious tinkering to lighten the trigger pull, and get rid of unwanted features such as the magazine disconnect.
At one point I decided to change the factory grips to walnut. No problem finding Ruger walnut grips for the Mark I and Mark II, but for the Mark III, I had to alter a set of Mark II grips. For the Mark IV, it proved impossible to find factory walnut grips that would be consistent with the previous ones.(Laminates are just not the same as solid walnut.) After trying several things, I went back to the plastic grips for the whole series.