Rare dime unnoticed for decades expected to fetch over $500K - is one hiding in your wallet?

A rare dime which was hidden for decades is expected to fetch half a million dollars at auction - but Americans are unlikely to find a similar one in their wallets.

The coin, which was struck by the US Mint in San Francisco in 1975, is one of just two coins known to exist which are missing an 'S' mark for the city. 

The other one sold at auction in 2019 for a huge $456,000, and then again days later to a private collector for $516,000, AP News reported.

Ian Russell, president of GreatCollections, the specialist which is handling the auction, thinks the coin could fetch $500,000 - or more. 

'The coin market is strong and there's been a lot of interest in this coin since we listed it a few days ago,' he told Fox Business. 

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

In 1975 the mint in San Francisco made more than 2.8 million uncirculated 'proof' sets which featured six coins and were sold for $7. 

It was not until a few years later that collectors noticed that two dimes from the set were missing the 'S' mark.   

Three sisters from Ohio inherited one of the rare dimes after the recent death of their brother, Russell told AP News, after it was hidden in a bank vault for more than 40 years. 

The sisters, who have chosen to remain anonymous, said that their brother and mother bought the coin in 1978 for $18,200, which amounts to around $90,000 today.

Their parents, who operated a dairy farm, saw the coin as a financial safety net, they told Russell.

The coin is now being displayed at a coin show in Tampa, Florida, before the California-based online auction closes in late October. 

The bid on the dime is up to $280,000 as of Thursday, September 12, with 45 days left to go. 

But for those hoping to find a rare coin in their pocket change, Russell has bad news.

While there is a chance more examples of the rare dime are out there, they would only be found among the 1975 'proof' sets.

'If you have old proof sets at home – perhaps your granddad or dad ordered them and forgot about them. If you have 1975, check the dime,' Russell said.

Last year, Russell revealed to DailyMail.com the most valuable pieces he has ever sold at auction - including a $12million silver dollar and a set of Lincoln pennies which fetched $7.7 million.

He developed an interest in rare currencies in his teens, and now typically sells up to 5,000 coins a week.