Since the release of Rowling's Black family tree (auctioned off), it has become more and more probable that the mysterious R.A.B. is Regulus Arcturus Black. It just fits. However, you gotta be suspicious of this, because Rowling never makes anything so obvious... but who else could it be? I suppose I'll believe this theory, but something tells me we're probably in for a surprise.
... R.A.B. is probably Regulus Arcturus Black, Sirius' younger brother, who was killed at the age of 18, presumably by Voldemort himself.