As i was re-readeing Harry Potter 6 i think I have found out who R.A.B. is. Im not sure though. R.A.B., I think, is Regulus Black. Sirius said Voldemort killed him but how could he be sure. Think, Sirius said his brother wanted to back out, he also said that he THINKS Voldemort killed him. But if Regulus escaped after almost getting killed he might want revenge. What better than to find his weakness(horcruxes) then steal it and destroy it.