welll if it was either one, it would be marked on the right side of the receiver,Mark I OR Mark II, if it is NOT 'marked' then its either a 'standard auto,RST-4 or RST-6 ' depending on the barrel length,fixed sight guns back in the early days are NOT marked, the adjustable sight guns will be........yes, either of the Mark I or later Mark II, double slot magazines will work in any of the olde r models, just have to reverse the follower button to which ever frame your gun, may be.....if the grip panel screw hole is LOWER ( closer to the bottom) of the grip frame on the LEFT side of the gun, then it is an A-54 frame, the first one, later, they reversed this hole location , flopped it over to the RIGHT side being lower, then that is an A-100 frame, easy to tell no need to even remove the grips............
