Colt SP1 A1 Upper and Knights Armament M5 Rail Adapter System (RAS)
An overview with pictures

The M5 Rifle RAS (Rail Adapter System) converts AR-15 and M16 rifles with 20 inch Mil-Spec barrels into the MWS (Modular Weapon System) configuration. The KAC (Knights Armament Co) rail system forends do not require the AR15 to be modified. They are very simply installed in place of the plastic handguards with simple tools. The lower rail assembly is easily removed without tools for cleaning or for attaching a grenade launcher (M203)

The M5 Rifle Rail Adapter System (RAS) has a top and bottom half. Like the 'Triangle Handguard' the M5 Rifle RAS has a metal heat sheild, but because the M5 has more air circulation, it needs less protection.

M5 Rifle RAS (Rail Adapter System) with Knights Armament rail cover on top

Knights Armament M5 Rifle Rail Adapter System