Undercover Address – Post Office Box 506, Lisbon

Probably the least secret of all World War II undercover addresses was Post Office Box 506, Lisbon, Portugal, operated by the Thomas Cook & Son Ltd. travel agency of London. July 1940 press advertisements in Britain stated that mail service would be available to Belgium, Czechoslovakia, Danzig, Denmark, German-occupied France, Germany, Holland, Italy and Italian colonies, Luxembourg, Norway, German-occupied Poland, and the Channel Islands. The list was later expanded to include Bulgaria, Estonia, Finland, Greece, Hungary, Latvia, Lithuania, Romania, and Yugoslavia. The Cook firm collected a fee for the service, and mail handled through the system was subject to British censorship in both directions, but in a way that would not be revealed to the Axis countries’ postal authorities and censors.