A 2013 reprint of the Inverted Jenny stamp. (U.S. Postal Service)

In 1918, an offshoot of the federal government — the Bureau of Printing and Engraving — made a specific and, in the scheme of Washingtonian blunders, relatively minor mistake.

It misprinted a batch of 24-cent stamps.

Specifically, a batch of stamps featuring the Curtiss JN-4D, a biplane nicknamed the Jenny. Due to a printing error, the plane was flipped upside down, as though caught in the middle of an aileron roll. The Inverted Jenny, as the stamps would come to be known, swiftly turned into one of the most coveted stamps among philatelists, exploding in value over the next century: A single stamp, for instance, fetched more than $1 million at an auction in May.

Four other Inverted Jennys vanished in 1955, stolen from a stamp convention in Virginia.

The Internet ascends and the age of snail mail wanes, of course, but Americans still use massive amounts of stamps. The Postal Service delivered some 20 billion letters, bills, get-well cards, birthday notes and other pieces of mail requiring postage stamps in 2015. Why the Inverted Jenny was able to float above this billions-deep sea of stamps in the first place is thanks to one man: stamp collector William T. Robey.

On May 14, 1918, Robey arrived at a Washington, D.C., post office for the express purpose of buying a sheet of Jenny stamps. He did not know, not yet, that they would be inverted. But Robey did know that there was a good chance the Bureau of Printing and Engraving had goofed. The Jenny stamps were the Bureau’s second attempt at printing stamps in two colors — a blue plane in the center of a red border.

“Printing a stamp in two colors back then was tricky business,” Scott Trepel, stamp collector and owner of Siegel Auction Galleries, recently told CNN. The previous time the Bureau attempted a two-tone stamp, in 1901, it “messed up,” he said.

What exactly happened when a clerk handed over a sheet 100 stamps to Robey, at a post office on the corner of New York Avenue and 13th Street, is unclear. But according to a 1938 account of the events retold in the Smithsonian Magazine, the clerk “brought forth a full sheet,” Robey said, “and my heart stood still.”

The clerk, the Smithsonian noted, was clueless: “How was I to know the thing was upside down?” he reportedly said later. “I never saw an airplane before.” The Curtiss JN-4D had been invented the year before, and though it would become famous for delivering mail by air, the Jenny’s inaugural postal flight was May 15, 1918, one from New York and another from Washington, landing in Philadelphia.

But when Robey asked for another sheet, the jig was up. In 1918, spending $24 on stamps was surprising, and $48, suspicious. The clerk closed down shop, and no one else would ever buy a sheet of Inverted Jennys.

After refusing to hand over the sheet to postal inspectors who showed up at his apartment, Robey flipped the sheet to a Philadelphian, Eugene Klein, for $15,000, according to the Smithsonian. Klein, in turn, resold the Inverted Jennys for $20,000 to railroad scion Edward H.R. Green. Green divvied the sheet up into 25 rectangles of four Jennys apiece.

From there, the Jenny sets passed through various hands. Green almost burned some in a pub ashtray before fellow stamp collectors stayed his hand. He numbered the stamps in pencil, 1 to 100. One stamp was stolen from the New York Public Library; another Inverted Jenny possibly appeared on a 2006 absentee ballot.

The mail-in stamp “made all the news headlines because people thought it was real,” Trepel told CNN, “but anyone who knew anything knew it was a forgery.” About 98 of the 100 stamps, he says, have been accounted for.

Thievery is responsible for those that are missing. In 1936, Ethel B. Stewart McCoy bought a quadruplet of Inverted Jennys. In 1955, she allowed the American Philatelic Society to display the stamps at a Virginia convention. That was the last time those four stamps appeared together, as they were filched from the premises. Twenty years later, Chicago collectors found two of the four. But, after the early ’80s, there was no sign of the missing stamps, except, perhaps, on the ballot.

In April, however, everything changed. Another stamp had been found, having made its way across the Atlantic years before. An Irish man, Keelin O’Neill, knew he had a strange stamp among items he had inherited from his grandfather, but did not recognize its significance.

“I had no idea about the history and importance of the stamp until very recently,” he said to the Associated Press. When he tried to sell it to a U.S. auction house earlier this year,  the assessor thought he had a forgery.

“The chances of him having the real McCoy, so to speak, were between slim and none,” George Eveleth, who evaluated O’Neill’s stamp, told NBC News. But when it turned out to be real, collectors tipped off the FBI.

Missing #InvertedJenny Returns to the Stamp Library.

Posted by World Stamp Show-Boston 2026 on Thursday, June 2, 2016

On Thursday, during the World Stamp Show in New York City, the FBI handed it over to the American Philatelic Society. For his part, O’Neil turned out okay, scoring a $50,000 reward for his role in wrapping up a part of the mystery.

But the final Inverted Jenny remains at large, just as the question of who pilfered the block of four of the rare stamps, 60 years ago, remains unanswered.