I don't know why but I have this crazy notion that R.A.B could well be Aberforth Dumbledore.
I have searched high and low, left, right and centre across the net looking for his full name (like how Dumby's is Albus Percival Wulfric Brian Dumbledore). Cannot seem to find it!
Somehow maybe his full name could have the remaining initials of 'R' and 'B'.
So then this was the note that was found alongside the locket:
To the Dark Lord
I know I will be dead long before you read this
but I want you to know that it was I who discovered your secret.
I have stolen the real Horcrux and intend to destroy it as soon as I can.
I face death in the hope that when you meet your match you will be mortal once more.
R.A.B.
When it states, "I will be dead long before......" - this can only mean that the person is indeed still alive! Dumbledore's brother is still alive and working as the barman of the Hog's Head.
(This excerpt is taken from Wikipedia) It may be possible that Aberforth is currently in possession of the real version of the fake Horcrux found by Harry Potter and Albus Dumbledore at the end of Half-Blood Prince. Leaving Mundungus in Hogsmeade in Half-Blood Prince, he was described as "drawing his cloak more tightly around his neck" as he walked away. Since the Horcrux is a locket, it's possible that he purchased from Mundungus the unopenable locket found by Harry and the others at Grimmauld Place in Order of the Phoenix. If this is the case, it raises further questions as to why Aberforth did not inform his brother about the locket, or, if he did, why Dumbledore went to the trouble of acquiring the fake. It is expected that we will learn more about Aberforth in the seventh and final Harry Potter book.
(Back to me now) I just think that Regulus Black just seems TOO obvious. I just feel it is gonna be someone who we never expected.
What do you guys reckon? 😕