Okay my run down on R.A.B.
I agree J.K. usually isn't obvious ... but R.A.B. makes sense to be Regulus Black.
Recently rereading the 5th book it makes a lot of sense. Obviously the quote from page 116 "also a heavy locket that none of them could open" seems to scream horcrux. It is probably the least obvious scene in any of the books. They're cleaning and J.K. focuses more on the metal contraption crawling up Harry's arm and the music box. I have little doubt in my mind that locket is the horcrux.
I guess the question a lot of people are asking is why would Regulus have it. The note left in the locket is not one of protection or admiration. It's revenge. Whoever wrote it wants Voldemort to die. Why would Regulus want Voldemort dead? Well considering the fact that Voldemort ended up giving the word to kill Regulus there could be the assumption that he somehow knew he was going to be killed, OR Voldemort may have found out that Regulus intended on destroying the horcruxes. Once again though that theory brings us back to why Regulus would want Voldemort dead.
Assuming Regulus knew he was going to be killed, that's pretty self-explanatory in my book. I would want to get revenge on someone who planned to kill me too. But if there was another reason, why then?
The Black family was all about pureblood. That's why they supported Voldemort. However, many of us realize a) Voldemort himself was not pureblood and b) Voldemort was only after power ... obtaining a pureblood wizarding society was more of a way to cope with his own tragic childhood and neglectful pureblood father. Sirius points out on 112 regarding the pureblood family's following Voldemort "They weren't alone either, there were quite a few people, before Voldemort showed his true colors."
How did Regulus get the locket? This question came up in another thread and I found one explanation that made a lot of sense. He took Kreacher with him. He forced Kreacher to drink it, which could also explain why Kreacher has certain mental disabilities. Now I'm sure there are a million possibilities to this scenario. The Kreacher one just made the most sense to me.
Although Regulus seems to obvious to be R.A.B. my certainty that the locket they find on 116 is the horcrux leads me to believe that it had to be Regulus. Then again this is why J.K. is such a great writer, I wouldn't put it past her to put a red herring in a book ahead of time just to throw off crazy conspiracy theorists like myself. The final question though is ... where is the locket now?
I have two possible yet vague places the locket could be both involving Kreacher. "Several times, Kreacher sidled into the room and attempted to smuggle things away under his loincloth, muttering horrible curses every time they caught him." Now people will throw this away right away saying he was never successful, but those words were never written. He attempted, there were probably times he succeeded and times he failed. In either case that starts my premise to the location of the locket.
The easy route. Kreacher still has it in Number Twelve, Grimmauld Place.
The difficult route. When Kreacher left to see Bellatrix Lestrange he took it with him, meaning Voldemort once again has posession.
I'm always open to new theories as long as they make sense ... plus a good argument always makes you think harder haha. With that being said, poke holes in my theory.  | 
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