First Generation Buffer Tube (Receiver Extension) for Colt sliding stock models: XM177, XM177E2 and Colt Commando Models up to the 1980's.
Original Colt variants were first 2 position - just like this one.
In the 80's it was replaced by the 4 position
Most current builds now use a 6 position
2 Position Mil-Spec size
4 position Commercial Size
6 Position Mil-Spec Size