A block of stamps that were mistakenly printed with the illustration upside down have been sold for $2.7m (£1.52m).
The stamps were sold before the error was spotted
The sum paid at auction for the stamps, which date from 1918, is the largest amount ever paid for US stamps.
The stamps bear a picture of a Curtiss JN-4 plane, known as a Jenny, which was used for training pilots in World War I and later became an airmail plane.
About 700 of the erroneous stamps, now called an Inverted Jenny, were printed but only 100 went on sale.
Once the mistake was spotted by postal inspectors the rest were withdrawn from sale.
The entire page of 100 stamps that did go on sale was snapped up by stamp collector William Robey in 1918 at a post office in Washington DC.
It was later broken up into smaller blocks and individual stamps to be resold.
The block of four, which was bought by an anonymous buyer on Wednesday, is the only one in existence that shows the printing plate's number upside down and in the stamps' margin, Scott Trepel, president of Siegel Auction Galleries, said.