HS 7/49 Special devices

Door key. A door key has been successfully adapted for the concealment of small microprints. The shaft of the key is drilled to about three quarters of its length and a small stud is made which fits into the hollowed shaft by means of a left-handed thread. See illustration.

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What was the key used for? Why would this have been a handy tool for a spy? A micro-print contains sections of printed text that need to be magnified in order to read.