TOLEDO, Ohio (AP) -- Three sisters from Ohio who inherited a dime kept in a bank vault for more than 40 years knew it had some value. But they had no idea just how much until just a few years ago.

The extraordinarily rare coin, struck by the U.S. Mint in San Francisco in 1975, could bring more than $500,000, said Ian Russell, president of GreatCollections, which specializes in currency and is handling an online auction that will end in October.

What makes the dime depicting President Franklin D. Roosevelt so valuable is a missing "S" mint mark for San Francisco, one of just two without the mark known to exist. The other one sold at a 2019 auction for $456,000 and then again months later to a private collector.

While serious coin collectors have long known about the existence of these two rare dimes, their whereabouts had remained a mystery since the late 1970s.

"They were hidden for decades." Russell said. "Most major collectors and dealers have never seen one."

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.

The sisters from Ohio who inherited one of those two dimes after the recent death of brother want to remain anonymous given their sudden windfall, Russell said.

They shared with Russell that their brother and mother in 1978 bought the first error coin discovered 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.

One of the sisters said her brother often talked about the rare coin. But she never saw it first-hand until last year.

Russell, whose company is based in Irvine, California, said their brother reached out to him about seven years ago and eventually told him about the coin. He too kept the secret.

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?"

Now the coin, known as the "1975 'no S' proof dime," will be displayed at a coin show beginning Wednesday in Tampa, Florida, and before the auction closes in late October, Russell said.

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 expects this latest discovery to set off a lot of searching.