Originally Posted by
Selicate
I have a new in box PG 1050 12-25 to look at, there is no metal spacer used anywhere and all of the plastic ones are much thicker than what you are describing. On the one I have here, that metal ring could only come from the inner side of the 12t or 13t cog breaking off. It may be that the piece you have didn't originally come with the cassette.
Thanks for checking your PG-1050 12-25. You may be right about the paper-thin shim/spacer not being part of the original cassette. (When I bought my bike, I had the dealer swap out the original 11-32 cassette for a 12-32 cassette and they may have added the shim in order to provide "better" shifting.)