Six decades after it was stolen from a display case at an American Philatelic Research Library show, a rare “inverted Jenny” stamp with an estimated value of $350,000 was returned by the Justice Department in a ceremony at the Javits Center on Thursday.

The 24-cent postage stamp was issued to celebrate the start of airmail service in 1918 and displayed a Curtiss JN-4H “Jenny” biplane, but one 100-stamp sheet was mistakenly printed upside down, making the error stamps a well-known prize for collectors.

The recovered stamp, one of a block of four on loan to the Philatelic Research Library that was snatched during a 1955 show and broken apart, resurfaced in April when it was consigned for sale to the Spink auction house, triggering an investigation by the FBI and the office of Manhattan U.S. Attorney Preet Bharara.

Keelin O’Neill, the British man who consigned it to the auction house, said he had received it from his now-deceased grandfather in 2013 and knew nothing about its origin, officials said. He agreed to its return, and is now in line for a $60,000 reward.

But the original thief remains a mystery.

“There were no witnesses, no suspects and little evidence to pursue,” said Diego Rodriguez, head of the FBI’s New York office.

The four-stamp block, according to a news release from the library, had been loaned by Ethel McCoy, a wealthy collector and the daughter of Charles Bergstresser, a founder of Dow Jones & Co. It was stolen during a show in Norfolk, Virginia.

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“The McCoy block was deftly plucked from the Norfolk exhibit in broad daylight as the show prepared to open for the day,” author George Amick wrote in a 1986 book about the stamps.

“The thief cut a cord binding two of the exhibit frames and slid back the covering sheet of glass several inches. Armed guards had been stationed in the exhibit hall. A suspect has never been named.”

McCoy left title to the stolen stamps to the library, located in Bellefonte, Pennsylvania, when she died. Two of the stolen stamps were recovered during the 1970s. One is still missing.

Bharara, in a statement, encouraged anyone with information about the last stolen stamp to come forward.

“We hope that someday soon we can celebrate the return of the final missing McCoy Inverted Jenny as well,” he said.