rhatland, the R6721 is the AR15A3 Tactical Carbine. It came with a 16" HBAR, chrome lined, a 1 in 9" twist barrel, flattop upper, A2 "birdcage" FH, bayonet lug and collapsable stock. An R6721 lower will have a serial number of BD000134 or below, if it is a pre-ban.
To the best of my knowledge, the pre-ban rifles were manufactured in 1994, before the September, 1994 ASW bill. After that, they became an item for LEO's only.