Harry Potter Question: Who is RAB and where is the real locket?

RAB is Sirius Black's brother Regulus Arcturus Black. He had the real locket and I think he gave it to Kreacher and then it was stolen from Grimmauld Place by Mundungus Fletcher. He was selling it and then Umbridge came along and threatened to arrest him for selling stuff illegally but then she took a fancy to the locket and got it. Harry, Ron and Hermione go to the Ministry Of Magic in disguise (Polyjuice Potion) and manage to get it off her.