If the Mk II mag doesn't fit then try reversing the magazine follower button to the other side of the mag. This is a common issue with the Standard Model (pre Mk I).
Below is a picture of my RST4 Standard (A54 frame) and next to it is a Ruger Mark III magazine. The follower button has been moved to the right side of the mag. body.........and it works. The magazine latch wants to catch in the step between the floorplate and mag. body, but by pushing the latch a bit further back and it will come out just fine. I also have a Mark I Target (A100 frame) and in both of these pistols, I find that the Mark II magazines work better than the originals.