Little did they know that the rare coin, struck by the U.S. Mint in San Francisco in 1975, held immense value. Ian Russell, the president of GreatCollections, a currency specialty company handling an online auction, revealed that the dime could fetch over $500,000 at the upcoming auction, which will conclude in October.

What makes this dime depicting President Franklin D. Roosevelt so remarkable is the absence of the "S" mint mark for San Francisco, making it one of only two known to exist without the mark. The other dime without the mint mark was auctioned in 2019, fetching a staggering $456,000 before being sold again to a private collector. Despite the existence of these two rare dimes being well-known among serious coin collectors, their actual whereabouts remained a mystery since the late 1970s.

"They were hidden for decades," noted Russell. "Most major collectors and dealers have never seen one." In 1975, the San Francisco mint produced more than 2.8 million uncirculated "proof" sets, each featuring six coins, which were originally sold for $7. It wasn't until a few years later that collectors discovered two dimes from the set were missing the mint mark.

The sisters, who wish to remain anonymous following their unexpected windfall, inherited one of these elusive dimes after the recent passing of their brother. The significance of the coin became apparent when they shared with Russell that their brother and mother purchased the first error coin discovered in 1978 for $18,200, equivalent to roughly $90,000 today. With their parents operating a dairy farm, they viewed the coin as a crucial financial safety net.

One of the sisters mentioned that their brother often spoke about the rare coin, but she had never seen it in person until last year. Russell, who was contacted by the brother seven years ago, had also kept the secret. When he finally revealed the potential value of the coin to one of the sisters a few years ago, she was astonished, exclaiming, "Is that really possible?"

The "1975 'no S' proof dime," as it is now known, will be on public display at a coin show set to begin in Tampa, Florida, from Wednesday. Following the exhibition, the dime will be put up for auction, with bidding closing in late October, according to Russell. While there is a slight chance that additional examples of this rare dime exist, they would only be found among the 1975 "proof" sets and not in everyday pocket change, clarified Russell.

This incredible discovery by the Ohio sisters not only unveils the hidden value in inherited possessions but also highlights the allure and excitement of rare coin collecting.