Hi! Please accept my belated welcome to B&O!
It's a theory, but there is at least one major question there: Other than the "A" in the middle, how do the other initials match up to Aberforth? Being Dumbledore's brother, his last name would also be Dumbledore. So far we have Aberforth Dumbledore (A.D.), and either way, there's not a "D" in the initials of R.A.B.
I think it's fairly clear that R.A.B. is Regulas Black, the brother of Sirius. I'm not sure, but I do think I remember reading that Regulas got in the Death Eaters group, but wanted to get out, and one doesn't just hand in a resignation letter to Voldemort. That would fit in nicely with what appears to have happened with the necklace. It would make sense that someone in Voldemort's inner circle who wanted out would have known they would be killed rather than just be allowed to walk away. Knowing what the locket represented, he would have done what he could to thwart Voldemort, knowing that he would die regardless. Anyway....
I think it's most likely that Aberforth is the owner and/or barkeep of the Hog's Head. 1. Dumbledore says that he goes there because he knows the owner (okay, I admit, this one I'm not 100% sure of. I'm PRETTY sure, though, that I read that). 2. The Hog's Head smells faintly of goats, and Aberforth once got in trouble for performing unacceptable magic involving a goat (and really, how much are goats mentioned in such odd circumstances???). 3. Harry notices that the barkeep of the Hogs Head seems familiar, but not as someone he actually knows, but more as someone he knew a long time ago, or as in a dream. To be honest, it's been a long time since I've read the books (I'm currently in the middle of something even more intriguing, so after I finish this one, I plan to go back through the Harry Potter books again). I've also somehow lost about 73% of my brain cells when I used to post, so my memory that used to be sharp as a tack is now about as sharp as a lump of clay. I think, though, that my argument about Harry's thinking he'd seen the bartender before was either that he had already seen the picture, or that the fact that he knew Dumbledore so well made him look at Aberforth (who could definately have very similar features of Dumbledore) and feel like he kind of knows him. It's happened often that people have had that kind of feeling looking at me if they know my sister, or vice versa.
Anyway, sorry for the rambling, and for any inaccuracies THAT I DON'T NEED PEOPLE TO POINT OUT. If you've been here long enough to know me, you know that it's rare that I'm totally off on what is said or done in the books (although my point of view can definately be off).
I could be right. I could be wrong. I'm okay with either.
And really, welcome to B&O and keep posting!