Certainly PMD since the coin has been taken out of round by the pressure indicating that the weirdness took place after the retaining collar was removed.
Not the least bit confident in my conjecture, but it looks to me like a lead bullet struck the coin. The unspent energy in such a bullet would spend itself as heat, enough heat to fuse the bullet to the coin.
I agree with all of the above, and I think Coinfrog hit on something, it does look like some solder on the reverse. Definitely post mint damage ( PMD) regardless how it was done.
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