Who is R.A.B?

In Harry Potter and the Half Blood Prince, there's a mysterious
character called R.A.B who claims to know the secrets of You-Know-Who.
In a message to the dark lord he claims to have stolen the real
Horcrux ( the locket of slytherian ). The only name I can figure out
is Regulus Alpherd Black who is Serius's uncle. You can find his name
in the fifth book ( Order of Phoenix ). Check out the Black's family
tree in the head quarter of the order. If this post interest you, feel
free to leave a comment.