An export/commercial variant of the Colt Model 629 rifle, the Model 639 closely emulates the "XM177E2" designated variant associated with Military Assistance Command-Vietnam Special Operations Group (MACVSOG) operators in the late 1960s, with a short barrel and telescoping buttstock for compact size and a proprietary sound moderator/flash hider designed to alter the report of the carbine and maintain proper back pressure to cycle the action. This variant was manufactured in very limited quantities and subject to increased scrutiny after the muzzle device was designated as a silencer by the BATFE; most owners discarded the device and didn't bother registering it, but this one has the device in place (see Item B for more info). Elevation adjustable post front and windage adjustable 2-leaf peep rear sights, with "C MP C" on the underside of the barrel, no bayonet lug, a round ribbed handguard, and a "C" and "H" marked upper with short casing deflector and teardrop forward assist. The lower receiver has the "AR-15" logo and "MOD.639" markings on the left side of the magazine well, with a fenced magazine catch, checkered pistol grip, one-piece buffer and a two-position telescoping buttstock. Included with the lot is a Beta Company C-Mag double drum magazine.

Developed in response to soldier feedback on the CAR-15/Model 607, the XM177 flash suppressor was intended to reduce the intense muzzle blast created by running 5.56mm rifle ammunition through a short barrel, as well as provide additional back pressure to assure function of the gas mechanism. Overall length is about 5 inches, with laser engraved serial number on the side and an integral "birdcage" flash hider at the end. Internally, a short segment of perforated tubing vents into a small internal expansion chamber; it is primarily this feature that makes it a silencer in the eyes of the BATFE.