(no subject)
So, I think R.A.B. is Regulus. I also think the locket is at number 12 Grimmauld Place. I think this proves it:
...while they emptied the glass cabinets that afternoon... They found... a heavy locket that no one could open
-OotP page 116, US edition
"See this?" [Gaunt] bellowed at Ogden, shaking a heavy gold locket at him..."Slytherin's ... Salazar Slytherin's!"
-HBP page 207, US edition
I can't take full credit for this discovery though. It was a friend who pointed out the OotP bit after I'd mentioned that I thought the locket was at #12. & then I had to go look it up for myself. Remarkably similar wording describing it in both books.