I guess the real question now is.. Who is R.A.B? The only name I thought of was Regulus Black. The problem is, he's dead. So here's my theory. If R.A.B is Regulus Black, then his things might still be at 12 Grimmauld Place - unless Mundungus Fletcher stole it. Now and then through the book people would talk about how shifty he is, and then there was the confrontation in Hogsmeade. I don't think all of that would be in the book if it wasn't pretty important. JKR seems to have cut out a lot of superfluous stuff in this one, apparation lessons for example.
Theories aside. Would Ron and Hermione just hurry it up already? I mean, seriously, could there be any more tension between the two of them?