The only known die that exists for Byzantine coins is coming back on the market.

Classical Numismatic Group offers the die in its May 10 Internet and mail-bid auction.

The piece is the only die extant for some 1,000 years of Byzantine coinage, according to the auction house. The iron die would have been used to create the reverse of a bronze follis coin of Justin I, who ruled from 518 to 527. 

“Iron dies are by their very nature a rare and valuable commodity and, in most cases, they would have been melted down and reused,” according to the auction house. “There are only about a hundred Roman dies extant, mainly of the first century (bronze or iron).”

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The die being offered by CNG weighs 790 grams and measures 88 millimeters long. Its diameter is 35 millimeters at the die face, widening to 40 millimeters at the center, and tapering to 33 millimeters at striking face. The die is “as made, shank rusted and pitted.”

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The die was used at the Nicomedia Mint, under the third officina’s workshop. Since the officina letter on this die has only been recorded on a small number of coins, “This may go some way to suggesting that some event, administrative or otherwise, could account for the fact that this die somehow escaped destruction.”

The die previously sold in 2002 during CNG’s Triton V auction, where it realized a hammer price of $28,000 against an estimate of $10,000, the same estimate for the 2017 auction.