while in the marines in the mid 80's, we were the last to use the tri-guard M-16's, i always liked them better than the newer round guards.
last year, CDNN ( and cheaperthandirt) had some "new,found in warehouse, colt AR uppers with tri-guard and forward assist" for $369. i bought one,then got a rock river fully assembled lower and BCG for, IIRC, the same price. soon after i traded it away, and just the other day bought it back with almost a case and a half of ammo,( mostly XM193) many mags,a bayonet, a scope and tactical rail, slings, cleaning kits and i can't remeber what else......all for $800 !! win some. lose some !!!
sorry, didn't mean to sidetrack. if you're interested in these,though, check out AR-15.com, they have a retro section devoted to the old-style rifles.