I got this reply from sawsales about the AR6551 and the MT6551 differences.
Both of those models are now out of production at Colt. The AR6551 is an LE only gun since the 1994 ban. It is essentially a Colt M16A2 rifle that is semi-auto only. All parts are the same as the M16A2 except for the lower rec., fire control parts, and bolt carrier. It has a chrome-lined 1/7 BBL and US Mil-Spec parts except for the semi only fire control system. The MT6551 is a civilian, post ban Match Target Rifle. It has no bayonet lug, no BBL threads, no muzzle attachments, no chrome lining, 1/7 twist BBL. It also has a plastic buffer assy and a plastic buttstock door assy. The MT6551 is a cheapened, politically correct, post ban version of a real rifle!