Overprints
In 1916, at the request of Brigadier General Edward Northey, to the Governor of Nyasaland, Nyasaland stamps were overprinted "N.F.". The overprint was intended to be "N.F.F.", for "Nyasaland Field Force", but the telegraph operator omitted one “F.” when sending the request to the Governor. The stamps could only be used by troops of the Nyasaland Rhodesian Field Force. Although they were primarily intended for use in German East Africa, they were also used from field post offices in Nyasaland and Mozambique. They were not issued to any civilian post office nor could they be used by any civilians.
The civilian population were able to send mail through the Indian Army postal service field post offices using Indian Expeditionary Forces stamps (Indian stamps overprinted "I.E.F."). When civilian post offices were opened in 1917, stamps of East Africa and Uganda were issued overprinted with "G.E.A.". The same overprint appeared on stamps inscribed "East Africa and Uganda Protectorates," but these were issued after the establishment of Tanganyika, and are considered part of Tanganyika's postal history.
Read more about this topic: Postage Stamps And Postal History Of German East Africa