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During World War II, the card company Bicycle secretly worked with the U. S. government in fabricating special decks to send as gifts for American prisoners of war in German camps. When these cards were moistened, they peeled apart to reveal sections of a map indicating precise escape routes. Also during the war, USPC provided “spotter” cards, which illustrated the characteristic shapes of tanks, ships and aircraft from the more powerful countries. The company further assisted by sewing parachutes for anti-personnel fragmentation bombs.

-via hellsinky