Little marks usually detract from the value of coins, but one very significant mark on an early American silver dollar transforms the coin into a major rarity.

A 1799 Draped Bust silver dollar with an octagonal countermark depicting English King George III resulted from a brief emergency effort to meet the unquenchable demand for silver coinage in late 18th century England. One of five or six examples known, the coin highlights Davisson's auction No. 33, which closes Jan. 22.

The piece, formerly in the Emery May Norweb Collection, is in Good Extremely Fine, a very high grade for the coin, according to Allan Davisson. It was purchased from Spink in 1957.

A void of silver coinage in circulation in the latter third of the 1700s forced England to use paper money and merchants to issue an abundance of trade tokens.

The Bank of England in 1797 began adding an oval countermark to foreign silver coins, mostly Spanish 8-real coins from the Spanish-American mints of Mexico City, Lima and Potosi, according to Peter Seaby in The Story of the English Coinage.

The Spanish coins were lighter and contained lower fineness of silver than the earlier British crowns (the countermarked coins traded as 4 shillings and 9 pence), so they had less silver than their face value, discouraging the hoarding. The American silver dollars have slightly more silver than the Spanish-American 8-real coins but still less than their new face value as countermarked.

According to Spink's Nov. 19, 1986, catalog of the third part of Norweb's English collection, five examples of the octagonal countermark are known on a U.S. silver dollar. A possible sixth piece has since been discovered.

To read the complete article, see:Auction offers 1799 silver dollar repurposed as British coin(www.coinworld.com/articles/auction-offers-1799-silver-dollar-repurposed)