This was one of the famous 24c U.S airmail stamps issued in 1918. Yes that’s right the face value was 24c which was actually quite expensive in those days.
There was only ever one sheet of 100 of these stamps, the inverted error, sold to the public. The post office did find some others but they were defaced and then shredded to prevent profiteering.
I won’t go into the complete story because a book and many articles have been written which detail its fascinating history.
It was sold by the auction firm of Robert A. Siegel Auction Galleries, Inc. of New York City on behalf of the owner. In 2005 they also sold a plate block of four of the same stamp for a record of $2,970,000 for a U.S philatelic item. How much higher can these go? Only time will tell. Read more at the Robert Siegel Auction website:
The sale catalogue (sale #946A) has wonderful write-ups on the history or the invert error by Scott R. Trepel (“The Allure of the Inverted Jenny”) and a great introduction by Joe R. Kirker ((“…and my heart stood still”). The catalogue can be downloaded in PDF format.