TO rare tell me featuring a portrait of President Franklin D. Roosevelt just sold for more than $500,000.

Struck in San Francisco in 1975, the coin was part of a large collection of “proof” dimes. It was also one of two known examples from that set to contain an error: a missing “S” mint mark, which made the coin a valuable collector’s item.

On October 27, three sisters from Ohio, who owned the dime jointly, sold it through GreatCollectionsan auction house that specializes in valuable currency.

“This is a very exciting coin for our company to auction,” Ian Russell, president of GreatCollections, tells Newsweek‘s Natalie Venegas. “We’ve handled many trophy coins over the years, including two 1913 nickels and two 1804 silver dollars—but this is the first time for the 1975 ‘no S’ proof dime.”

In 1978, the three sisters’ mother and brother bought the coin for $18,200, amounting to roughly $90,000 today, according to a statement from GreatCollections. When their brother died, they inherited the coin, which had been stored in a bank vault for more than 40 years.

The family had viewed it as a “financial safety net” for their dairy farm, per the Associated Press. One of the sisters tells the news agency that their brother frequently spoke of the coin, although she hadn’t seen it in person until recently.

Mint marks are letters that indicate where a coin was produced. For example, the Mint’s Philadelphia branch once marked their coins with the letter “P,” while some coins made at the West Point Mint are marked with a “W.”

“(Mint marks) hold the maker responsible for the quality of a coin,” according to the United States Mint. “When the US used precious metals such as gold and silver to make circulating coins, a commission evaluated the metal compositions and quality of coins from each of the Mint facilities. The evaluations ensured that each facility produced coins to the correct specifications.”

In 1975, the San Francisco Mint produced a “proof” set of more than 2.8 million coins. Three years later, collectors discovered that two of the dimes in that set were missing the “S” mark, making them a “modern rarity,” according to the lot listing.

These two coins are the only 1975 “no S” proof dimes that are known to exist, although it’s certainly possible that there are others, as Russell tells the AP. In 2019, the other known copy was sold for $456,000. The buyers, a group that included coinage expert Mitch Spivack, resold it for $516,000 five days later.

“It was a great honor for me to own the finest known 1975 ‘no S’ dime, even for just a few days,” Spivack told Numismatic News in 2019. “As the owner of the unique 1976 ‘no S’ silver Eisenhower dollar for many years now, … I always dreamed of also owning the 1975 ‘no S’ dime. “That dream became a reality for me!”