I have absolutely no idea who RAB is, but many ideas come and go. Regulus is a pretty good bet, I think. Is Arturus Black dead? I don't know about Filch. Just doesn't seem to fit the role. Too fidgety. It might have been someone in Tom Riddles year at Hogwarts. Malfoy has Harry. Snape had James. Tom had ....? Harry's Grandfather?