The Viet Nam war era lasted a long time. We had advisers in VN as early as 1962/63 with prototype ARs. The war itself ran from 1965-75 during which the AR/M16 was constantly updated and upgraded.
Forward assists and chrome lined bores came about because early ammo used the wrong type of powder and rifles were not issued with cleaning kits. They tended to jam up.
I have an SP1 upper that I returned to it's original configuration by adding the triangle hand guards and three prong FH. I added an A1 stock and pistol grip to my lower. The lower is not correct in terms of pins and raised fence around the mag release but it still has the feel of an early rifle.
The lower rifle is the SP1 in original configuration. Note the lack of a forward assist.
The top rifle's upper (forget about the lower) is the next phase. It has A1 sights and a teardrop forward assist. Either triangle hand guards/three prong FH (early) or round handguards and bird cage FH (later) are appropriate for this version. Round hand guards eventually replaced the triangle ones however.
The middle rifle has the A2 rear sight and a round forward assist.
If you really want to get into the retro look this website has all the specs