The original was the CAR-15 "Commando" with the 10" barrel, a 3-prong flash suppressor, collapsible stock and triangular handguards.
The Air Force wanted one, so they created the CAR-15 "Survival Rifle" which retained the 10" barrel, a shorter pistol grip, a cone shaped flash suppressor, a fixed, tubular buttstock and rounded handguards. Due to the huge fireballl when fired, the Air Force decided not to adopt them.
Colt then created the XM177 which had a 4.5" flash suppressor (which ATF decided was also a sound suppressor), 10" barrel, collapsible, tubular buttstock and no bayonet lug.
When the forward assist was added to the M16, it was also added to the XM177, which was then designated the XM177E1.
About a year later, the barrel was lengthen to 11.5" and it became the XM177E2. It remained in this configuration until it was replaced in the 80s with the M4.