Bidding on a 1792 silver centre cent stands at $500,000 ahead of Heritage Auctions' August 5-9 auction of US coins in Chicago.

The lot is among the rarest and most historically important coins ever issued at the Philadelphia Mint as it was the first to be designed and struck in house.

The 1792 silver cent was the first coin to be struck in the Philadelphia Mint

The silver plug in the centre of the coin was used as a way of minimising the size of the issue, as the Mint act called for the value of the metal used to be equal to the value of the coin.

Had pure copper been used, this would have resulted in a coin the size of a half dollar.

The new pattern was struck on December 17, 1792 - an event Jefferson related to Washington in a letter the following day: "Th. Jefferson has the honor to send the President two cents made on Voigt's plan by putting a silver plug worth ?� of a cent into a copper worth ?? of a cent.

"Mr. Rittenhouse is about to make a few by mixing the same plug by fusion with the same quantity of copper.

"He will then make of copper alone of the same size, and lastly he will make the real cent as ordered by Congress, four times as big."

The example offered is the second finest of the 12 or so that are known to have survived.

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