The M16A1 was the same as the original M16, except for the addition of the forward assist, "fence" around the magazine release, chromed bore/chamber, and new birdcage flash hider (which was closed on the ends to prevent it from catching or snagging things), and 1/12 twist rate for better stabilization of the M193 cartridge in colder weather.
The chrome plated and slick-sided bolt carrier was replaced with a parkerized "notched" bolt carrier that worked with the forward assist. It was standard issue from about 1967 through 1985 or so.
Initial models usually featured 20 round mags. It was later replaced by the M16A2. Some units today still get stuck with old M16A1's, and this is the reason 62gr M855 has a green tip, to distinguish it between the 55gr M193 that is meant only for use in the older M16's with 1/12 and 1/14 twists.