The whole RAB thing has everyone guessing and this person could be anyone for all we know as JKR sure has the ability to keep us guessing till the very end. But this one sounds the best of all the versions that I have heard(it is convincing,though the ultimate test will be cleared only when the series ends) RAB could be Regulus Black...the trend has been to have a middle name after father for the eldest man carrying the name Black,and father's younger brother for the son next in line and so on,so Sirius had an Uncle Alphard and that could explain the A.Plus it seems less likely that Regulus,son of the Blacks,one of the most ancient and pure blood families(and he was no squib,in fact talented,Slughorn says so)was killed personally by Voldemort just for getting cold feet.Voldemort must have suspected that he had an idea about something... Also,Fred and George have come up with their best ever-U-NO-POO-that IS so funny!!!
