I have one of the first generation mkII 22/45, the ones with the molded zytel nylon frame. Will all 22/45 mags fit this model? I’m a little confused on what mags fit what guns. I would think they would all be backwards compatible.

I have a 22/45 Mk II. The Mk III mags will work, and so will the Mk IV. If you are concerned about maybe that not being true, take the gun to a LGS  and ask if you can open a package to see if a Mk III mag will fit. They let me do it. The Mk III mags seem to have better factory springs, IMO.

Just so you know, a lot of Mk II parts have been discontinued by Ruger. You may wish to stock up via Wirthwein or whomever.

I believe the only difference is that newer mags have a notch for the revised release button. The notch won't affect use in the mki or mkii