Well, thanks to this merry band of enablers I made the trip and scraped the last remnants of the gun fund of just enough to bring it home. Here it is:
The serial number dates it to 1997, and the Colt website says it is an MT6551.
The collapsible stock is a Rock River replacement. If anyone wants to swap this for a full length fixed one, let me know.
It was either rack number 6 or 9. Probably 6.
I like the 20" barrel and the long round handguards. Since I think of everything in terms of movies, it reminds me of Three Kings.
The barrel is marked like this. No bayonet lug as I had originally thought.
The innards look normal.
Carrying handle is fixed, with these sights.
I didn't have an AR, but I had some magazines in my orphaned mags box. One is dated also to 1997.
Now I either need to get the muzzle threaded for a flash hider, use a bolt-on one, or just get used to its weird appearance.