I had a Colt Sporter Target Model R6551, they have a prefix serial numbers of ST and I bought mine brand new when they first came out in 1991.
The uppers were 20" barrels government profile under the handguards, A2 carry handles, top of barrel in front of the front sight base are marked "C MP 5.56 NATO 1/7", A2 flash suppressors, except they don't have bayonet lugs.
My particular lower was an early production with large pivot pin, large hammer & trigger pins, have fence around the mag release button, A2 fixed buttstock, but no protruding steel sear block in the lower receiver, the later versions had sear blocks.
The Sporter Target Model replaced the former model Government Model rifle R6550 20" barrel (1991 or even late 1990), then the Target Model was replaced by the Match Target series during the start of the assault weapons ban of 1994.