Whose initials where on the orthodox r a b in Harry Potter half blood prince?

Answer 1

The initials written on the note that was inside the fake Horcrux are R.A.B. It stands for Regulus Arcturus Black, Sirius' younger brother who was a death eater that died young. It turns out what Regulus betrayed his master once he found out about the Horcruxes and later died stealing the locket.

Answer 2

The initials that were found in the note in the locket were RAB.

Answer 3

R.A.B. was Regulus Arcturus Black.