If the government never intended for the coin to leave the mint, then the government has a claim on it forever.
It is just like if you find a sunken Spanish Warship from 1650 loaded with gold, the Spanish Government will lay claim to 100% of the treasure. That is why, if you are diving and you find a Spanish warship, you never tell anybody about it.
Likewise, if you happen to find a Denver mint 1974 aluminum penny, you’d better not try to sell it in the United States.
I can imagine some unusual scenarios in which the man might legitimately have the penny, but they require us to imagine a time when that Mint actually thought those coins were going into circulation.
Theoretically, we can maintain that the government never intends mis-stamped or badly stamped coins to leave the mint. Therefore, valuable deviant coins belong to the Mint as coins that were supposed to be thrown away.
In any case, I agree with you. Shut up. Find a buyer and bank your winnings. That’s what I’ve done with all the bitcoins I’ve found buried in my back yard.