I had this on my Colt M4 but never used the rail for anything other than a forend grip. I used it for one range session and prefer the regular forend. It comes with the box and instructions. $150 shipped.
Here is Surefire's description of it. It slides on and is tightened by two screws that when you tightened them down they stay. I guess I need to take a pic of it again but it's like new.