An extraordinarily rare dime whose whereabouts had remained a mystery since the late 1970s has sold for just over $500,000.

The coin, which was struck by the U.S. Mint in San Francisco in 1975, depicts President Franklin D. Roosevelt and is one of just two known to exist without its distinctive “S” mint mark.

Three sisters from Ohio inherited the dime after the death of their brother, who had kept it in a bank vault for more than 40 years.

The coin sold for $506,250 in an online auction that concluded Sunday, according to Ian Russell, president of GreatCollections, an auction house based in Irvine, California. The auction house said there were 212 bids placed on the coin.

The only other known example of the “1975 ‘no S’ proof dime” sold at a 2019 auction for $456,000. Shortly after that, the coin sold for $516,000 to a Roosevelt Dime collector, who currently has the only complete collection of the series, the auction house said.

The mint in San Francisco made more than 2.8 million special uncirculated “proof” sets in 1975 that featured six coins and were sold for $7. Collectors a few years later discovered that two dimes from the set were missing the mint mark.

Russell said the sisters from Ohio, who wanted to remain anonymous, told him that they inherited one of those two dimes but that their brother and mother bought the first error coin discovered in 1978 for $18,200, which would amount to roughly $90,000 today. Their parents, who operated a dairy farm, saw the coin as a financial safety net.

When Russell told one of the sisters just a few years ago about the coin’s potential value, he said she remarked “is that really possible?”

While there is a chance more examples of the rare dime are out there, they would only be found among the 1975 “proof” sets and not in anyone’s pocket change, Russell said.

Still, he said he expected the latest discovery to set off a lot of searching.