Working swiftly and silently, someone cut the rope securing the leg of the display case and inched it forward. A sheet of protective glass was slid back, and four rare stamps were plucked from their display frame.

Minutes later — around 9:30 on a September morning in 1955 — a delegation of esteemed philatelists strolled down the row of display cases, looking expectantly for the star item of the collection: a block of four famous 24-cent stamps with the airplane in the center printed upside down in error. The stamp is known to collectors as the Inverted Jenny, after the nickname of the Curtiss JN-4 biplane.

But the block was gone. The Federal Bureau of Investigation interviewed the armed guards and others in the room and came up empty, unable to even name a suspect.

In the nearly 60 years since that theft, two of the stamps have been recovered, but the other two remain lost. Now, a prominent stamp dealer is offering a $100,000 reward to try to help close the case.