during the 1941-1945 participation of the U.S. in WWII, the U.S. armed forces established Army Post Offices (A.P.O.) in the countries in which they were stationed. Country of origin correspondence from these offices is identified by the A.P.O. identification number within the cancellation, and is further defined by the cancellation date; there is but one entry for Aden: A.P.O. No. 663 dated 28 August 1943-30 June 1945. A complete listing of U.S. Army Post Offices identifying the origin of the A.P.O. Nos. is available in the following reference: Forte, Jim, and Richard W. Helbock, A Price Guide to U.S. A.P.O. Cancels of the Second World War, Lake Oswego, OR: La Posta Publications, 1996