As the auctioneer's hammer fell in New York on Wednesday night, yet another philatelic record tumbled.
A block of four stamps, depicting a First World War trainer airplane, the Curtiss JN-4 or "Jenny", had been coveted by collectors worldwide. At $2.7m (£1.5m), the anonymous buyer became the proud owner not only of the most expensive block of American stamps ever sold, but the second most valuable philatelic item on record, according to Bolaffi International Catalogues in Turin, which produces an annual index of auction prices.
The stamp market has been on an upward curve in recent years, according to Hugh Jefferies of Stanley Gibbons. But what makes a stamp valuable goes beyond mere rarity. The Jenny commanded such a high price because it was printed upside-down. Many others in the top 10 contain similar errors.
And just as the bidders waited for this week's auction, so they must wait for the other stamps they know to be out there. Anticipation is mounting that some of the rarest and most sought-after are to come on the market very soon.
"The best-known stamps tend to only turn up at auction every 30 years, and no one knows how long one individual will hold on to an item," said Mr Jefferies.
"What the collectors are after is a stamp where the story is known, which has been in some of the most famous collections. By buying them, their name will live on for as long as people continue to collect stamps.
"When you have two real collectors and only one item, they are going to throw everything at it."
Stamps are considered to be the fourth safest investment - on a par with land and property. Some have more than doubled in price in six years.
The strongest markets tend to be in developed countries, such as Britain, France, the US and Australia, with British stamps particularly sought after.
Curtiss JN-4 'Jenny' 1918
SOLD FOR $2.7M IN 2005
Seven hundred stamps featuring the First World War training aircraft were printed upside-down. The inspectors caught all but 100 which were snapped up by the collector William Robey. These included the block of four sold yesterday, which also feature the inverted printing plate number in the stamp's margin.
Penny Black, 1840 with letter
SOLD FOR $2.4M IN 1991
More than 60 million Penny Blacks were produced as the world's first official adhesive postal stamp. Many still exist in private and public collections. This example is the earliest known and the first used on a letter. It was cancelled after being sent five days before the official launch of the stamp on 6 May 1840
US Franklin Z-Grill, US, 1867-68
SOLD FOR $935,000 IN 1998
The rarest of all US stamps, only two are known to exist. They feature Benjamin Franklin and are embossed with the distinctive Z-Grill designed to prevent fraud. The design was cancelled soon after this.
Alexandria Blue, US, 1846
SOLD FOR $1M IN 1981
Can expect to shoot up in value if, or when, it ever returns to the market after a 24-year absence. Issued by the state of Virginia, it precedes the first official US government stamps by a year.
Hawaiian Missionaries, 1851, on letter
SOLD FOR $2.09M IN 1995
The first stamps to be issued by Hawaii were on very poor quality paper and few survived. The letter sold in 1995 commanded a particularly high price because it contained four stamps. Named after the missionaries who used them to send letters back to the US.
Baden Green 1851
SOLD FOR $1.7M IN 1985
Having been off the market for nearly a generation, philatelists are keenly awaiting the day it will return to auction. Preceding unification under Bismarck by two decades, this nine-Kreutzer stamp was mistakenly printed on green paper.
Mauritius 1847 - Mauritius Blue or Post Office
SOLD FOR $3.8M IN 1993
A watchmaker's error on the British colony of Mauritius created the most expensive set of stamps and cover to be sold at auction. Instead of engraving the "Post Paid" he etched "Post Office". Some 200 were issued and Lady Gomm, the wife of the governor, used them to frank her invitations for a fancy dress ball.
Tete-beche France 1849-1950
SOLD FOR $1.1M IN 2003
Examples of this block of one-franc stamps with distinctive upside-down "head to spade" features twice in the most expensive 20 stamps sold. Unused, it commanded a record price for a French stamp at auction in Paris.
Tre Skilling Banco, 1855
SOLD FOR $2.3M IN 1996
Despite erroneous reports that it was a fake, this unique Swedish stamp, probably used on a double-weight letter in Nya Kopparberget, was sold twice at auction in the 1990s, achieving vast prices both times. Found in a loft by a 12-year-old boy, it was mistakenly printed on yellow rather than green paper. The eight-skilling banco stamp is on yellow paper.
British Guyana - One Cent Black on Magenta
SOLD FOR £850,000 IN 1980
Considered the world's rarest, it is expected to become the most expensive should it return to the market. The stamps themselves were issued by the postmaster of the British colony of Guyana while waiting for new supplies to arrive. Very poor in quality, this unique example was also discovered by a 12-year-old boy in an attic.Reuse content