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.