(UPI) A rare U.S. dime missing its mint mark was auctioned for $506,250, nearly 30 times the amount it sold for 46 years earlier.

GreatCollections, which handled the auction of the "1975 No S Proof Dime," said the coin was purchased by an Ohio collector and his mother for $18,200 in 1978, and it remained with the family for nearly 50 years.

The collector's family decided to have the dime auctioned, and it was given a Proof-67 grade by the Professional Coin Grading Service before fetching a sum of $506,250.

"We received interest in this modern rarity from all over the world -- serious collectors from Germany, Japan and the U.K., as well as collectors from the U.S. -- over 400 unique bidders were actively tracking the auction," GreatCollections President Ian Russell said in a news release.

The dime is notable for its missing "S" mint mark, which would signify it was minted in San Francisco. It is only one of two examples known to still exist.

"This is the Grail of modern coins, one that is missing from the Smithsonian, ANS and ANA institutional collections. After spirited bidding, it was ultimately won by a long-time client of ours who appreciates rarities that infrequently appear on the market. His goal is for his family to own it for the next 46 years, similar to the seller's family who consigned it to GreatCollections," Russell said.

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