According to Colin Walker's Mafeking Siege Register, "Brown, A, Pte, 182" served with the "Cadet Corps and Bechuanaland Rifles" (or is that the CC of the Bech'land Rifles?). Joined 4/2/00 age 12. Attended 20 drills, Grant paid, 5/6. Also some other info re sporting achievement. QSA clasps "DofM" and "Transvaal".
I did not have time to look at the relevant rolls, unfortunately. Only Colin Walker's excellent reference. Clearly, the number did come from the BR. The way Colin recorded the units meant they could have been either! I very briefly did wonder at the "Transvaal" clasp - but if young Cadet Brown qualified for a DofM, then equally it would have been possible to be awarded "Transvaal" as well.