I know that some people think that R.A.B. is not Regulus Black. I, though, truly think it is. He is mention 3 separate times in this book, as well as in the last book. Since the Horcrux was stolen by someone who Voldemort would know by their initials of R.A.B., it almost has to be either someone famous enough to be known by their initials, which we should have mention of somewhere but don't, or a Death Eater that left, which is the most plausible because of the note and familiarity. Regulus Black we know left the Death Eaters, so he would be the perfect candidate for this, and the initials seem to fit. Also, his death soon after leaving could be directly related to his theft of the Horcrux, probably from the potion (which fits with the whole "Good Snape, Dying Dumbledore" theory). One final piece of possible evidence is that in the fifth book, while cleaning Grimmauld Place, they find "a heavy locket that none of them could open" in the cabinets. If Regulus made it back there (or maybe he had help getting it in the first place, since it seems to need two people...Sirius?) he could have hidden it among his parents' collection, and there is every chance that his parents could have turned him over to the Death Eaters.