1972 Nickel, Reverse Error Or PMD

my first thought was that it was glued as well. But notice the spalding to the left. I just can't for the life of me figure out how it could have been done during the minting process. I am inclined towards PMD, but that spalding has me scratching my head. But if it was hammered into it, then why is the obv unaffected?

looks like a post for a lapel pin or maybe an earring.

I think the foreign object appears to have a raised bump in the center and the base is obviously of some thickness and a step up off the surface itself. So this looks like something ( a retained struck through ) that is 3-D.
I don't think that it could be a mint error in any way as the surrounding detail would have to be either missing or weak and the bump in the center would have to be flattened out.

Now if that design is part of the nickel itself then I would think somebody at the mint took something like a blade drill bit to the reverse die that made it. That would be strange but there would probably be more out there and this 1986 Guide Book/39th Edition ( Maybe a 1986 Red Book ) will explain more.