purpose of two-way communication mirror

[jakedjensen]

No: HPFGUIDX 135967

That silly mirror that Sirius gives HP in OoTP has been the source of 
a good deal of discussion. Given what we now know, I think the mirror 
has the following purpose:

Regulus Black has the other mirror (perhaps found it in Grimauld 
place...perhaps he had another...perhaps he even bought it off 
Mundungus) and will use it to contact HP in book 7. 

Jake

To summarize my previous posts:
R.A.B. = Regulus Alphard Black (numerous reasons to think this)
The horcrux locket is the locket noone can open in Grimauld Place in 
OoTP.
Mundungus now has the locket (but likely doesn't know what it is), 
hence the briefcase scene in HBP (where we find out MF has been 
stealing from Grimauld place again).