The attestation papers for Scotts Railway Guards are in WO126/127-129 at the TNA Kew, the nominal rolls are also to be found there in WO127/20, some have been reproduced on this site, in the column on the left, unit information, South African units and then Scotts Railway Guards, the QSA roll is on line at ancestry
I checked the QSA roll for Richard Robert Cooper and there was a note in the remarks column, "305 WLH also 1267 JMR"
So this fellow served at some point with the Johannesburg Mounted Rifles and is on their nominal roll in WO127, in addition, he also served with the Western Light Horse, enlisting at Kimberley on the 18th February 1902 and serving with that unit until it was disbanded on the 30th June 1902.
To conclude, I would think a visit to Kew would be the way to go, as you may find more papers in both WO126 and WO127.
Good luck in your research Karin,