I think you have answered the most important question, already-you have no idea what kind of life the gun led before you got it.
What you have done is normal use, with good maintenence. Little chance for wear with the use you descripe.
That said, there is no knowing if the frame is sprung without some serious measuring-it could be uneven wear on one, or both ends of the hook allowing the shim into the right side only. I would bet the frame isn't sprung. Wear to the pin and lump have occured exactly as dictated by the design of the gun. The single, non selective trigger has chosen where the wear happens.
Have it fixed, and carry on. Blessed are those who get the opportunity to wear one out, and have life left to put it right and try to do it again.
I'll bet you can't wear it out again, however.
Hey, can I interest you in a design that has a zero % chance of coming off face?
Good luck.
Best,
Ted
