The Vietnam and 1970s/early 1980s era M16/M16E1/M16A1 styled rifles look like mine below and Madcratebuilder's above.
Nobody makes complete rifles like this any more. I had to build mine with New Old Stock parts. The furniture, 1/12 twist barrel, and upper are all surplus M16 or M16A1 parts. But still my lower doesn't look right (it's a DPMS A2 type lower with an M16 stock and grip).
This is the M16A2 style (Panama/Gulf War I/Somalia era) that most ARs are patterned after today. Most obvious is the thicker barrel (at the muzzle anyway, a real M16A2 barrel is still thin under the hand guards), different carry handle, different sights, round handguards, longer stock, even the pistol grip is different. The rifling twist also changed from 1/12 to 1/7.