NEW YORK (AFP) - An incredibly rare 19th-century postage stamp, a tiny one-cent magenta from British colonial Guyana, goes under the hammer in New York on Tuesday, valued at between US$10 million and US$20 million (S$12.5 million and S$25 million).
Sotheby's says the current auction record for a single stamp is US$2.2 million and that, if it sells, it will become the world's most expensive as well as most celebrated stamp.
Made in 1856 in Guyana and measuring just one inch by 1.25 inches (2.54cm by 3.18cm), it is octagonal, printed in black ink and bears the initials of the postmaster.
Mr David Redden, Sotheby's director of special projects, has described the stamp as having "extraordinary fame and charisma" and being in remarkable condition given it is more than 150 years old.
Last bought by convicted murderer and American multi-millionaire John du Pont in 1980, it was last seen in public in 1986 before going on display at Sotheby's in the build-up to Tuesday's sale.
Hinged to a paper backing, since 922 it has already broken three times the record price of a single stamp sold at auction.
But Sotheby's says that if a secret reserve price agreed by the seller is not met, then the stamp will not be sold.
"If it sells, at this estimate, it will be the most expensive stamp ever sold," Sotheby's spokesman Darrell Rocha told AFP.
The auction house says the stamp is the only surviving example of a one-cent magenta, so rare that it is missing even from the British royal family's philatelic collection.
It is "the largest shining star in the very distance in the great universe of collecting", Mr Redden told AFP in February.
Colonial Guyana depended on supplies of stamps from England, but when a shipment was delayed in 1856 the postmaster commissioned a contingency supply.
The printers of the local Royal Gazette newspaper quickly ran off one-cent and four-cent magentas, and a four-cent blue.
The only surviving example of the one-cent was rediscovered in 1873 by Vernon Vaughan, a 12-year-old Scottish boy living with his family in British Guyana.
He found it among some family papers and added the stamp to his album. Vaughan then sold it to another collector for a few shillings and the stamp made its way to Britain in 1878.
It was bought by French Count Philippe la Renotiere von Ferrary, perhaps the greatest stamp collector in history, and later donated to a museum in Berlin.
After World War I, France seized his collection as part of war reparations due from Germany and sold the stamp in 1922 at auction to Arthur Hind, a textile magnate from New York.
Hind paid a then record US$35,000 for the stamp. It sold for a second record of US$280,000 in 1970 and was bought in 1980 by the late du Pont for a third record of US$935,000.
In a case that shocked the United States, du Pont shot dead Dave Schultz, an Olympic gold medal freestyle wrestler, at his estate in Pennsylvania in 1996 and died in prison in 2010.
The stamp is being sold by his estate. It has been on show at Sotheby's in London, Hong Kong and New York.
Current-day Guyana, which won independence from Britain in 1966, is a small but poor nation of about 700,000 people.
Sotheby's says the current auction record for a single stamp is US$2.2 million, set by the Treskilling Yellow in 1996.
US$4 million is the auction record for any philatelic item, set in 1993 by The Bordeaux Cover, which includes both of the "Post Office" stamps issued by British colonial Mauritius.