I appreciate the point you make Vince, but I did not think it was match ammo - that was suggested by Berdan III. Neither do I think it is likely to be “sniper” grade ammunition, as this was not made for British or Commonwealth service. Even if the South Africans had made something like that I think we would have heard of it by now.
Also, I must disagree with you that a black stained case simply means non standard. Black staining has been used in British service for two distinct reasons. It has been used to identify match ammo, although this was a very limited military use for a few years in the early 1900s and was really a civilian practice that continued for many years. The main use has been to identify discharger cartridges.
Military ammunition tends to be either experimental or issue, and the former is usually identified by coloured tips, stripes or whatever. I do not know what this round is, and I suspect we will never know, but given the date I don’t think it is as suggested.