] One cleverly concealed map scheme elicited the help of
] the U.S. Playing Card Company of Cincinnati, Ohio.
] Workers inserted numbered segments of a large escape
] route map into a full deck of cards for reassembly at the
] stalag. Each map segment replaced the card's opaque,
] black center layer, which prevents light from penetrating
] the card and divulging a backlit hand to an opposing
] player. The company printed an entire map onto a block of
] fifty-two cards, two jokers, and two company cards prior
] to cutting, a method that ensured a close fit of map
] segments. Workers affixed the face of the card with a
] water-soluble rubber-based adhesive that could either be
] peeled or soaked apart.

Picture of one of the cards here, under "P.O.W. Playing Cards"
 http://www.loc.gov/preserv/bachbase/bbcmaps.html

World War II -- Map Concealment in Deck of Cards