The Mark III 22/45 magazines have an additional notch protruding from the upper left side of the magazines. That notch acts to accept the rear "fin" on the back of the Mark III 22/45 triggers. The fin on the rear of the trigger engages that notch to ensure that the magazine is properly inserted, or seated, because of the magazine disco parts. Those who have installed a
Sam Lam hammer bushing, and removed those magazine disco parts, will no longer need that notch in the magazine, or the rear fin on the Mark III 22/45 trigger. Therefore you can use, or interchange magazines for the 22/45 or the Mark III 22/45 pistols. I do have some 22/45 magazines if anyone is interested.
See the notch on the magazine on the right? Both of these are Ruger Mark III 22/45 magazines. The Ruger 22/45 magazines don't have that notch.