The original M16 was a far cry from what you see today. The original M16 had a 1/12 or 1/14 twist, 20" skinny "pencil" barrel, three prong flash suppressor, shot 55gr M193 ammo, no forward assist, triangle handguards, short fixed stock, and chrome plated and slick-sided bolt carrier. Selector switch, Safe, Semi, Auto.
The one pictured here looks to have an M16A1 lower receiver, notice the fence around the mag release.
It was later replaced by the M16A1.