READ THIS READ THIS READ THIS!!!!!
This is some great evidence that R.A.B. is Regulus Black, Sirius' brother
"Further evidence can be gleaned from foreign-language editions of the books in which the Black family name has been translated. In the Norwegian edition, for instance, the Black family name is 'Svaart,' and the initials on the note have changed from R.A.B. to R.A.S.; in the Dutch version, the name is 'Zwarts,' and the initials are R.A.Z. The foreigh-language editions that kept the English surname of Black, however, retained the locket initials R.A.B."
-Mugglenet.com's What Will Happen in Harry Potter 7
Go out and buy this book if you want to know more!!!!