Project Mercury: 4-Cent “Man in Space” Stamp, Mint Block of 15.

An unused block of fifteen (3×5, with full right-margin selvage) of the 4¢ commemorative marking John Glenn’s Friendship 7 flight, the first orbital spaceflight by an American.

Designed by Charles R. Chickering — the Mercury capsule ascending against the limb of the Earth and a field of stars — and printed under tight secrecy at the Bureau of Engraving and Printing (the engravers were told to claim they were on vacation), the stamp is unique in postal history: it was distributed, sealed and unopened, to 305 post offices nationwide in advance of the flight, with postmasters kept in the dark about the contents of the locked mailbags marked “Classified Material.”

Only once word came from Washington, within the hour of Glenn’s successful splashdown at 2:43 p.m. EST on February 20, 1962, were the stamps unwrapped and put on sale — the first and only time the Post Office Department issued a previously unannounced commemorative simultaneously with the event it honored, held back specifically so nothing would need to be issued if the mission had failed.