Had to try it for myself... my Ruger 22/45 Mark II original magazine got damaged, and OEM Ruger mags (AP-10 / 90045) are apparently discontinued. Rumor had it that the OEM Ruger 22/45 Mark III mag (this one) will work in the 22/45 Mark II pistol... so I had to check for myself and bought one. In my gun anyway, the 22/45 Mark III mag does indeed fit and feed just fine in my 22/45 Mark II. YMMV, some settling of contents may occur during shipping...