During the Second World War, playing cards were slipped into prisoner of war camps because they looked harmless enough, but hidden inside them were escape maps, allowing captured airmen and soldiers to peel apart the layers and reveal useful geography once the card game was over.
I do wonder how opposing military authorities failed to spot this little arrangement, especially given that wartime censors were not always generally known for their relaxed attitude to suspicious parcels. Perhaps the cards looked too banal to deserve much scrutiny, or perhaps someone simply saw a pack of them and thought that prisoners deserved at least one modest pleasure before returning to the business of attempted escape.
Either way, I like the ingenuity and the museum thinks that these were produced around 1943. The International Spy Museum in the United States notes:
“To date, decks of these cards are said to have helped at least 32 people escape from Colditz Castle and prompted some 316 escape attempts.”
There were some very careless military authorities amongst the German troops letting this lot through.