Originally Posted by CHLChris
Is it the integral carry handle upper that makes it an A1? What is a flattop upper, then?
Not really since the plain ol' M16, the M16A1, the M16A2, the M16A3, a few early M4s and the ArmaLite/Colt AR-15 (pre-M16) all had integrated carry handles. Only the M16A4 and current M4/M4A1s are flat tops. Of the commercial versions, usually you hear A3 and A4 for flat top.
An M16A1 had a few upgrades over the M16 like the chromed chamber/barrel, forward assist, etc.