CIA shoelace communication

This is an illustration from The Official CIA Manual of Trickery and Deception, a book inspired by the spy manuals written for CIA by John Mulholland:

In 1953, the fledgling CIA hired professional magician John Mulholland to adapt his techniques of stealth and misdirection to the craft of espionage. Mulholland produced two illustrated manuals featuring a range of tricks from placing pills into drinks to stealing documents and avoiding detection. The classified manuals were believed to have been destroyed in 1973, but the authors discovered a copy in 2007 among recently declassified CIA archives.

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Update: More illustrations - here