In March 1947, they literally let a fortune slip through their fingers, when they handed the then 16-year-old schoolboy a copper penny as part of his change. Made in error by the U.S. Mint in Philadelphia, the penny will now go to auction in Orlando and is expected to sell for $170,000 or more.

It’s all thanks to a historical quirk: The penny is one of a few 1943 examples made in copper, at a time when the mint was supposed to only be producing zinc-coated steel to save copper for the war effort. Numismatists theorize that a few left-over copper blanks from 1942 were stuck in coin-making machinery and made it through production and quality control and out into circulation.

Intrigued by his new coin, Lutes asked the U.S. Treasury for guidance and was told it had not made any 1943 copper pennies. Luckily for Lutes, he kept it in his collection.

Over the next few years, a few other examples appeared on the market and dealers began confirming their authenticity. An urban legend arose that Henry Ford would give anyone who found such a coin a new car. Ford representatives told Lutes the legend was false when he wrote, but by 1958, a 1943 copper penny sold for $40,000, the equivalent of over $350,000 today.

As of the morning of January 9th, bidding on the Lutes penny was at $120,000. A 2010 auction on a similar, but one of a kind, penny raised $1.7 million. And earlier this year, Fortune reported on a century-old nickel that sold for $4.5 million.

The auction for the Lutes coin, of which 10 to 15 are believed to exist, closes the 10th.

Correction, January 11, 2018: The original version of this article, including the headline, overstated the auction estimate for the coin. We regret the error.