I've only got one, but I find them fascinating.
Let's take it a step farther back, the "d" used for penny in the nail sizes goes back to the Roman denarius.
Best bet is to post pictures and the weight. Odds are that it is environmental damage as explained in this thread.
They were very heavily minted. When I was a kid in the late 60's early 70's I had a book of magic tricks. One of the tricks was to have someone...
No that is just a zinc blister.
Great news, thanks for the follow up.
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