Technically, actual "M16" rifles do not ever have the forward assist, regardless of when they were made. The M16 was the official rifle of the Air Force, and they insisted on having no forward assist. They only changed this attitude with the adoption of the M16A2 in the mid 80's.
Conversely the Army insisted their rifles must have the forward assist. The rifles made under contract for them were designated XM16E1 or M16A1. And yes, they had the forward assist from day one, before the first US combat troops ever officially entered Vietnam.
Of course the US had all kinds of unofficial involvement in Vietnam before they officially sent in combat troops, and there was lots of use of commercial AR-15 rifles by special forces, military advisors, and the CIA, and these did not have the forward assist.
The Marines and Navy didn't much care one way or the other about the forward assist issue, and when they needed rifles they bought whatever was coming off the line that day, which of course was usually Army rifles, but not always.