TOLEDO, Ohio – An extraordinarily rare coin whose whereabouts have remained a mystery since the late 1970s has sold for just over $500,000.

The coin, struck in 1975 by the U.S. Mint in San Francisco, depicts President Franklin D. Roosevelt and is one of only two coins known to exist without the distinctive “S” mintmark.

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

The coin sold for $506,250 in an online auction that ended Sunday, according to Ian Russell, president of GreatCollections, an auction house based in Irvine, California.

The only other known example of a “1975 ‘S’ proof coin” sold for $456,000 at a 2019 auction and was sold again to a private collector months later.

The mint in San Francisco produced more than 2.8 million special uncirculated “proof” sets of six coins in 1975, which sold for $7. Collectors discovered a few years later that two pennies in the set were missing the mint mark.

The Ohio sisters, who wish to remain anonymous, said they inherited one of those two pennies, but their siblings and their mother bought the first faulty coin discovered in 1978 for $18,200, which is roughly equivalent to $90,000 today, Russell said. His parents, who ran a dairy farm, viewed the money as a financial safety net.