On 18 January 1916, a new system was introduced to reduced the usage of postage stamps, which were in short supply due to WWI.
Envelopes could be sent to Morogoro, Dar-es-Salaam, or Tanga to receive a prepaid franking mark. The mark consisted of the value and the official seal of the post office.
The envelopes could then be used at any local post office.
The last copy shown above is one of only a handful of known copies with black instead of violet seal.