It is a deteriorating die cap. The cap from another planchet is still on the die, but after each strike the thickness is thinning even more. Sometimes they rotate and make even stranger coins:

The first clue on the struck through a capped die is the rim alteration. It will look more like a saucer than a normal rim.
CoopHome:
What does a deteriorating die cap look like. First clue to identify these?
Edited by coop
04/16/2022 6:18 pm