It was known as Operation Cornflakes.
According to an entry on Wikipedia, "the operation involved special planes that were instructed to airdrop bags of false, but properly addressed, mail in the vicinity of bombed mail trains. When recovering the mail during clean-up of the wreck, the postal service would hopefully confuse the false mail for the real thing and deliver it to the various addresses."
It goes on to say, "The content of the mail often included copies of Das Neue Deutschland, the Allies' German language propaganda news sheet. The postage stamps used on the envelopes were forged 6-pf and 12-pf Hitler-head stamps intended to look identical to genuine German stamps of the era, though these forgeries were printed by the Office of Strategic Services (OSS). In addition to these two forgeries, the OSS printed another design of the 12-pf stamp not-too-subtly redrawn so that Adolf= Hitler's face shows his partially exposed skull. Also, the country identifier 'Deutsches Reich' (German Empire) read 'Futsches Reich' (lost, destroyed, or collapsed empire). Copies of this 'death head' stamp were inserted in the envelopes along with other propaganda materials."
Shown above, the original three OSS stamp issues printed for the operation.
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