On 26th March 1941 M.I.9, the branch of Military Intelligence that helped Prisoners of War, initiated a scheme to send maps secreted inside Board games such as ‘Monopy’.
Waddington [well known manufacturers of board games] already possessed the technology to print on cloth and made a variety of board games, packs of cards and so forth that could sent to the camps. They began by printing silk maps for supply to air crews, both British and American, and went on to conceal maps inside Monopoly boards, chess sets and packs of cards which could be sent into the prison camps. The whole business of making the maps was shrouded in secrecy and the letters do not tell the whole story.
The references to different coloured playing cards, for example, made in one of the letters, are not explained at all in the correspondence; many communications were by word of mouth and never written down for security reasons. A special code, which is described in another of the letters, was used to indicate to the Ministry which map was concealed inside a particular game so that it would be sent to a prisoner of war camp in the appropriate area. A full stop after Marylebone Station, for instance, meant Italy, a stop after Mayfair meant Norway, Sweden and Germany, and one after Free Parking meant Northern France, Germany and its frontiers. “Straight” boards were marked “Patent applied for” with a full stop.
See the full British Library article by Debbie Hall.