The U.S. mint switched from bronze to a zinc-coated steel in 1943 because of shortages during World War II.

However, during the switch about 10-15 pennies were accidentally struck from bronze blanks stuck in a trap door of a tote bin used to feed coins into the press machinery.

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The sale was part of the World’s Fair of Money, which featured $1 billion of old coins and money.

A penny minted in 1792 also sold for $211,500. The “Birch Cent,” named for the coin’s designer, had been lost for 130 years. It was last seen at auction in 1890.

“It’s kind of fascinating,” David Stone, a coin cataloger with Heritage Auctions, told the Denver Post. “It’s a physical link to the past. Maybe George Washington held this coin at one time. Our whole money system came from these coins, and it wasn’t a simple process. It’s just a really good story.”

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