R.A.B. ?

[Sages Baby]

No: HPFGUIDX 164072

  I just read the Order of the Phoenix and Half Blood Prince again
and
I had a question. While reading a few things stood out this time
that didn't the first time. Like when Harry was at Grimmauld Place he
and Sirius were looking over the Black Family Tree, Sirius was
talking about his brother Regulus Black. This was one of the things
that made me read an re-read the paragraph. (forgive me I am new at
this)

"Oh no" Said Sirius. "No he was murdered by Voldemort. Or on his
orders, more likely, I doubt Regulus was ever important enough to be
killed by Voldemort in person. From what I found out after he died,
he got in so far, then panicked about what he was being asked to do
and tried to back out."


The note left inside the locket was addressed "To the Dark Lord".
Hasn't it been said in the books that only the Death Eaters refer to
Voldemort as "The Dark Lord"

Could Regulus have been R.A.B.? Has this discussion been brought up
already? If so I would love a link. Thanks.