Before it was the M16 it was the AR15.
AR15 was the ArmaLite/early Colt model number. When the military adopted the design they named it the M16. Colt then used the AR15 name for their semiauto civvy model.
I would guess the rifle in the first pic was one of the early Air Force purchased select-fire AR15's that was overstamped with the M16A1 when this rifle got the A1 upgrades.
As a side note, my work pard is a retired AF E-8 police/security. The other day he brought in one of the arms manuals he got during/after boot camp around 1966. The manual included operating/cleaning of the new AR15 rifle. It was real cool. It talked about and showed the early featurs of the rifle.