Yes, it certainly spent some time in South Africa: pre-1910, Cape Govt marks on buttstock [and inverted marks, indicating, I assume, sold out of service though I haven't see this arrangement before]; post-1910 the U with a broad arrow of the Union of South Africa.
Out of interest, are there any other marks associated with Cape ownership, namely a crowned 3, or the initials K.T.?
The armoury re-barreling with a BSA commercial barrel is often seen on South African military Long Lees, and seems to date from the Great War, though I haven't seen it done to a carbine before.
One of the forummers, Terrylee, is the world expert on British arms in South Africa; if you PM him, he may know a lot more about this carbine. Do send any more pictures, including of the whole carbine.