Harry Potter theory: The initials "R.A.B.", used to sign the note in the false Horcrux in the sixth book, Harry Potter and the Half-Blood Prince, stand for Regulus Black, Sirius Black's brother. The "A" will turn out to be his middle initial. I am making no claim to any evidence outside of that in the books, for example statements from J.K. Rowling or others, so please do not quote this as fact. It is only a theory.