The German colony of Deutsch-Südwestafrika had been in existence since 1890. It was invaded by South African forces in 1915 and was governed by their military. At the end of the war it became a League of Nations mandated territory, governed by South Africa. It used stamps of South Africa from 1915 until 1923, when it issued its own stamps. Here's a few covers from this fore-runner period.
Firstly, here's a cover posted at Windhuk Army Base PO 6, 1 Jun 15, to Windhuk. No receiving stamp. Also with bilingual "Passed by censor / Doorgelaten door censor" cachet in blue.

Secondly, here's a cover posted at Mariental 10.2.16, to Windhuk, using a converted German canceller. Transit stamp of Gibeon 12 Feb 16 and received in Windhuk 14 Feb 16. Also with "Passed by censor / Doorgelaten door censor" cachet in violet.

Next, a cover posted at Windhuk 19.10.16, to Bern, Switzerland, using a converted German canceller. Received Bern 9.XII.16. Also with green Windhuk registration cachet, censor's stamp in violet and red censor label (Afrikaans on the reverse of the cover).

Now a cover posted at Okasise Rail 16 Dec 16, to Bern, Switzerland. Transit stamp at Okahandja 16 Dec 16 and received in Bern 24.II.1917. Also with censor's cachet in violet and red censor label (Afrikaans on the reverse of the cover).

Next, a cover posted at Kub 10.1.19, to Windhuk. No receiving stamp but transit stamp of Gibeon dated 11.1.19. With violet censor's "100 A" cachet.

Military censorship ended in 1919. Here's an uncensored registered cover posted in Grootfontein 23 Jul 20, to Windhuk, received 26.7.20. It has a very makeshift registration cachet.
