Terminology note:
Colt Automatic Rifle, or CAR-15 was a family of weapons introduced around 1964, loosely comparable to the Stoner 63 weapon system family. The CAR-15 family consisted of:
CAR-15 Rifle (AR-15, or M16) - Model 603 or 604 (the Model 604 has the forward assist, the 603 does not)
CAR-15 Squad Automatic Rifle - M1 - a heavy barrel version of the rifle with a detachable bipod, and fed from a 30-round magazine.
CAR-15 Squad Automatic Rifle - M2 - a heavy barrel version of the rifle with a detachable bipod, and fed from a belt.
CAR-15 Carbine (Model 605A) - A Model 604 with the barrel shorten so the flash hider was just in front of the front sight, the Model 605B was the same but based on the 603.
CAR-15 Submachine Gun (Model 607) - This is the model most people associate with "CAR-15", it had the 10.0 inch barrel with a shortened ("carbine-length") gas system with shortened triangular hand guards and a collapsible stock (the short plastic type). This is what would evolve into the Army's XM177E1.
XM177E1 - (Colt Commando) - An improved 607 featuring round handguards, aluminum (coated) two position stock, improved hand guard slip ring, and sound suppressor (aka "moderator).
XM177E2 - (Colt Commando) - same as above but with an 11.5 inch barrel to allow the mounting of an XM148 40mm grenade launcher. By the 1980's the Army had removed all the moderators, due to their ineffectiveness.
NOTE: The carbine and submachine gun versions' barrel configuration did not allow for the mounting of a bayonet, therefore did not have a bayonet lug on the bottom of the front sight.
NOTE: The "M177" was reserved for the submachine gun without the forward assist, but that was never actually used, the USAF going with their own designation, "GAU-5/A", therefor all XM177E2shad the forward assist.