3d Transvaal and 6d Union postage due stamps used in occupied German South West Africa

This post is a little off-topic; one of the stamps is a Colonial era 3d Transvaal postage due but the other is a 6d Union postage due. Both have been used in 1916 in German South West Africa, during the period of South African occupation, cancelled “Windhoek” with Putzel Type 8 obliteration.

The stamps have been in the collection of a third party for many years, but he has still been unable to find any information about whether others exist, how many of each duty were sent to South West Africa in the first place, and when.  I certainly recall having seen at least one of them (possibly the 6d duty?) in a major sale a few years ago which went for more than I bid for it, but neither of us can find out any more information. There are conspicuous gaps in the SG numbering system around these two duties.

The owner has recently published an identical request for help in the latest edition of The London Philatelist (Vol. 130, April 2021, p172), but as many of our members may not be a member of the Royal Philatelic Society of London, I would like to see if we can offer any help. If you have any information, please can you contact me in the first instance?