I was cracking apart a 1981 proof set, for a really nice coin, and then I spotted this interesting error. I don't know of too many proof set errors, especially in the first year they were making proof strikes.
"Discovery follows discovery, each both raising and answering questions, each ending a long search, and each providing the new instruments for a new search." -- J. Robert Oppenheimer
that's not an error, like biokemist said, it's caused by high pressure from mint machine. but I do think it has collect value rather than other proof cent.
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