My old room mate was in the Marine Corps in the early 80s and he told me his issued rifle was mix of slab-side M16and M16A1 with round hand guards and an A1 butt stock. He called it a "commemorative model" because it had no finish left on it and looked almost chrome in places. He also said he could turn it on it's side and shake the crap out of it and the take down pins would fall out.
I have a late mid 90s Olympic Arms A2 with a tear drop FA.