The way the obverses are worn around the rim mostly looks like they were ground or smoothed down to go into custom belt buckles back in the 70s. The bevel of the first one above looks like it was made to just fit into the bezel of the buckle. Has been a long time since I have seen a buckle like that fall apart or even a buckle like that but memory leads me to think that is what happened to these.
There are obviously tons of other possibilities, but that is my best guess from the pics. That means the "black" stuff could have been tannin from the leather tarnishing them?
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