Due to increasing scarcity of lower value stamps due to WWI, in May 1915, the larger post offices in German East Africa began using cancels to denote franking.
The majority consisted of a box frame and the phrase “Fr. It. Einn./Nachw. in/[Location]” (Postage paid/confirmation of receipt in/[Location]). Dar-es-Salaam and Wilhelmsthal used different cancels for a short period of time.
The signatures or initials of two postal officials were added, along with the value of the cancel — usually 2 1/2, 4, 7 1/2, or 15 Heller.