Warning: Last items in stock!
During World War Two, the Department 9 of the British Directorate of Military Intelligence (M.I.9 - Escaped British Prisoners of War/POW, Debriefing, Escape and Evasion) developed a great number of secret means to conceal tools and instruments in harmless looking objects for everyday use. The means the pilots carried with them during the air raid were called pre-capture. These comprised many items like uniforms or shoes, that could easily be converted in civilian plain clothes but also see saws, food, etc. This department also sent to the POW's in the Offizierslager, short Offlag called camps (using fancy organisation names) parcels with escape aid items (called post-capture).
About one in 5 parcel contained such materiel concealed in tooth-brushes, pencils, playing cards, etc. The "empty" ones were called dove and the "hot" ones naughty. You could buy imitations in the Imperial War Museum (but this link is now dead...)