Owners of U.S. dimes may want to check their pockets and piggy banks, as one rare $0.10 coin is expected to bring in over half of a million dollars by the end of October, and it can be viewed in person in Tampa, Florida this week.

The 1975 dime lacks the letter "S" signifying the San Francisco mint it was pressed at and was discovered by a California coin collector who purchased five 1975 proof sets from the U.S. Mint.

According to the coin’s auction listing, there have been six "No S" Proof dime errors mistakenly released by the U.S. mint since 1968. In those six times, dies were shipped to San Francisco and lacked the mint mark, yet were still used to strike proof coins.

The years that No S Proof Dimes were struck were 1968, 1970, 1971, 1975, 1983 and 1990. While there are two to three dozen known to be in existence since 1968, there are hundreds known to be in existence for the years 1970, 1971, 1983 and 1990.

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But for 1975, there are currently only two known No S Proof Dimes in existence.

One of those coins was discovered in 1977 by California coin collector Fred Vollmer, who purchased five 1975 proof sets from the U.S. Mint.

The next year, CoinWorld published an article about the 1975 No S Proof Dimes, sending coin collectors and dealers scrambling to see if they had one of the coins themselves.

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Vollmer offered the coin to collectors who had purchased No S Proofs from him previously, and one of those buyers was Ruth E, who purchased a 1968 No S Dime from Vollmer’s company.

Ruth E was the first to inquire about the coin and ultimately purchased the 1975 No S Proof Dime and held onto it for decades.

For the first time since 1978, the coin is up for auction by GreatCollections Certified Coin and Paper Money Auctions.

Ian Russell, the president of GreatCollections Auctions LLC, told Fox News Digital what makes this coin so rare is that all 2.8 million dimes produced from the San Francisco mint in 1975 should have a little "S" on the coin. But two were discovered 45 years ago without the S mint mark.

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"They were both discovered in Proof Sets – so that is the key. They were never issued in regular coins found in change," Russell said. "The Proof Sets were sold by the U.S. Mint in 1975 and shipped to people all across the country. Still, many exist in sealed shipping boxes."

How many are actually in existence is still unknown.

"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.

The bid on the Ruth E dime was up to $266,000 on Tuesday night, with 47 days to go until the auction ends.

Russell said he expects the dime to fetch over $500,000.

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"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 said.

Anyone hoping to set their eyes on this rare dime can visit greatcollections.com or stop by the Great American Coin and Collectibles Show this Wednesday-Friday at the Tampa Convention Center in Florida. For more information, visit www.gacc.show.