I think youre right... i understand the Filch theory.. but i think its soo far-fetched that its hard to believe...

but heres another thought im just going to throw out there (even if you dont like it)... everyone takes the letters as RAB maybe its BAR... maybe they put their initials backwards... so who knows... what im really here to say is something COMPLETELY different... so here goes... but i was reading (for the third time) OotP about three months ago... and realize that in grimauld place... there was an old locket that no one could open that they found when they were in the process of cleaning... im thinking that is the locket horcrux missing from the cave... i mean raB... B=Black... and its in the black house... i think it got to have SOMETHING to do with it... i mean JK Rowling is soooo creative... it would be just like her to slip something slight and innoticeable into her books to give us some type of clue... so yeah... thats my theory...