Slickside= no forward assist (or brass deflector). AR-15A2 Sporter IIs came in three variants, all with the A1 rear sight: no forward assist, forward assist alone, and forward assist plus deflector. By 1988 Colt finally began to produce a proper M16A2 equivalent, the AR-15A2 Government Model.
And then promptly started messing with the rest of the rifle. Big pins, no bayonet lug, internal blocks, etc. Be nice if Colt did a run of A4s now that the ban's off.