I still have the two original magazines for my almost 30 year old Ruger Mark II.
They have seen thousands of rounds through them and work perfectly. Why I decided I need two more is beyond me, but I did.
I bought two, brand new, factory magazines made specifically for the Mark II and neither one would cycle correctly. I contacted Ruger (they have great customer service) and they made a few suggestions but none worked so I sent them back. I had two brand new ones in about a week. These moved a lot smoother than the others but still did not feed well.
I ended up taking a new one and an old one completely apart and compared each piece. I had noticed that the follower seemed to protrude more from the body on both of the old magazines and less on both of the new.
I also noticed that there were fresh scuff marks on the forward lips of the new magazines.
When I physically compared the two magazine bodies, I discovered that the forward lips were at least 1/16th of an inch shorter on the old magazines.
I could find no real difference in the rest of the parts.
I put all of the old parts in the new body and all of the new parts in the old body. Now the new parts worked great (in the old body) and the old parts would not work at all.
I took out the old Dremel with the fine sanding wheel and reduced the lips 1/16th of an inch (both sides)
Now I have four perfectly functioning magazines.
I do not know if this is just something odd about my pistol or if the magazines are made to a different spec but it solved my problem.
Hopefully this may help someone else.