The last incarnation was designated E or F with all the improvements before the A2. Could not tell from the pic the FA and lower. Looked like a shadow or the reinforcement from the A2. The later ones were all M16A1 but little things like the trapdoor buttplate VS the flat plate changes things. I remember getting the stupid bulletins on improvements from 81-89 when I ran the arms rooms and that is how they called them.

The bulletin would have something about the improvement for M16A1c rifles to be upgraded to M16A1e.