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Possibly 16295 Gunner Thomas CLARKSON, Royal Garrison Artillery or, the man you have already discovered, 16295 Bombardier Collar-Maker John BANNERMAN, R.F.A., attached to 23rd Bn. Mounted Infantry.

I'm no expert on service numbers for the Royal Artillery / Royal Field Artillery / Royal Horse Artillery, but I suspect the fact that your knife is marked to the R.A. rather than the R.F.A./R.H.A. makes Clarkson the more likely candidate.

Not sure about the initials "W.S." Perhaps added by a subsequent owner .....

Hi, This is a very late response, however I have only just seen it. There is a lot of information on clasp knives of the 2nd Boer War on the CASK website (at www.CASK.info ) in the Collector Note "Clasp Knifes of the Boer War".
Lawrie