Can we move on to RAB now?????? It isn't Regulus Black, TOO EASY. It is RA Burke from Borgin and Burke's shop, whom we haven't yet met, nor do we know his first name yet we know Borgin's first name (but I can't remember what that is off hand without looking it up). Otherwise it is someone we have never met but my guess is Burke.
I think it is definitely Regulus. JKR made a point of mentioning Regulus' name several times in Half Blood Prince when she really didn't need to. It was quite anvil-icious.
We associate JKR with all sorts of elaborate plot twists, but when you break it down, she really does go for this straightforward. The HP universe has an iternal logic.
We have one book left - this is not the time to bring in an elaborate backstory about Borgin, Burke, or some other character that has only be referenced briefly.
We know the basics of Regulus' backstory. He was the younger brother of Sirius, and followed the family tradition of going into Slytherin House and upholding the "pureblood" ideals. Sirius didn't think much of his brother or his wizarding abilities, and we have to wonder if that didn't factor into Regulus' decision to become a Death Eater (I'll show my big brother!).
Regulus changed his mind and tried to leave the DE's when asked to do...
something. For this rebellion, he was, allegedly, murdered on Voldemort's command. But no body was ever found, as Lupin conveniently tells us in HBP.
Now honestly - JKR is not going to tell us all that about a character that also happens to fit the RAB initials perfectly - and then go on to make RAB some guy she has mentioned in passing.
This is what I think. Voldemort gave several of his horcruxes to trusted DEs to hide. He didn't tell them they were horcruxes, just something extremely valuable. Lucius got the Diary, Bellatrix got something (maybe the cup?), Voldemort keeps Nagini with him always, we still have the unknown horcrux with its unknown keeper,
and Regulus got the locket.
So Voldemort tells Regulus - hide this for me. Maybe even tells him exactly how to hide it, since Dumbledore says he senses Voldie's "style" in the various enchantments on the cave.
At some point in the course of hiding the locket, Regulus finds out what it is. He's horrified. He's all for pureblooded wizards, but the horcrux is the most evil invention of the wizarding world. He realizes that Voldie is looking for eternal evil domination and Regulus decides to rebel. So he plants the fake locket - which explains how he "got past" the enchantments, he didn't have to get past them since he set them up.
Now - Regulus has to figure out what to do, because he's dead as soon as Voldemort figures out what happened. Did he...go to Dumbledore and ask for help? And Dumbledore faked Regulus' death? When Dumbledore tells Draco he can fake his death, is it because he has done it before?
I think we will meet the not-so-dead-after-all Regulus in Deathly Hallows, and he will be RAB.
For that matter, I think that Snape's turning point has something to do with Regulus too. Snape loathed Sirius, and wouldn't it be a lovely revenge to bring Sirius' brother into the Death Eaters? Snape and Regulus would have been in the same House and attended school in overlapping years. So Snape befriends Regulus for the purpose of enraging Sirius, and maybe Regulus actually likes him - considers him the mentor that his own brother refuses to be, and then the kid is "murdered" so Snape thinks - after joining the DE at Snape's urging.
See, it always comes back to Snape!