The bayonet lug went away at the end of the Sporter 2 green label era, those initially had the Sp1 slabside lower mated to a field site A1 upper then to a A2 rear site upper of which there were several variations.
Can't recall if those ended with a fenced lower but they have the early SP1 large pivot pin with screw head
The one you are describing sounds like its a blue label 6601 "Sporter" series Hbar, early guns will have a threaded barrel with A2 flash suppressor, very early ones have a hole drilled in the raised "fence" on the right side lower receiver for a spring retained takedown pivot pin (although none came with those parts installed), Colt instead chose to use a flat head screw on the end of a non retained small diameter pivot pin.
While the pre ban blue label guns with thread on flash suppressors bring a pretty penny ( most I see go for $2k) they have the larger hammer / trigger pins, pinned in receiver sear block and cut bcg, they are well built rifles but not as good as the earlier ones,
The low point for Colt AR's were made in the Clinton AWB era and first had a pinned on muzzle brake then no muzzle device at all with those bringing the least of all variations
All early Colt ARs have good collectability , the post ban 6920 LE marked A4s are arguably the high water mark as all were built from milspec USGI parts.
Current production 6920 rifles look the same but are built from a mishmash of sub contracted parts that may or may not be milspec.
On a side not IIRC it was the entry of Bushmaster (Windham Ma) entering the arena and offering a true A2 copy that forced Colt to start eliminating all the Clinton era stupidity.
Here is a link to episode 1 of 4 that details the evolution,
The Colt AR-15 Sporter I Series Evolution - YouTube
The 6601 should be in the beginning of episode 3 here
Colt Sporter Series, Part 3 - YouTube
...Ben
