That is an A.R.M.S. S.I.R. System the model is I beleive #50C
I had a REAL one on one of my PTW's, very plasticky "bottom portion" and very bulky, just not my taste... There should be some clones out there
http://www.armsmounts.com/default.asp?mode=sir
The VLTOR CAVS-EL is very similiar to the S.I.R. system; futhermore, it is more sturdy...I had one of those also...
http://www.vltor.com/handguards_and_...pics.htm#CASV1
Both of these 2 systems are a very simple which are drop-in and tight the top rail on to your upper receiver....
edit" Sorry I forgot about that S.I.R. I had; I needed to disassemble my delta ring and use the barrel nut only...however, maybe there are clones that might allow you to have the rail system with the delta ring intact...