In early 1982-83, Aeromarine Inc out of Birmingham, AL, who was a Colt distributor, had several dozen M16A1 carbines and 11-1/2 Commandos (Model name only). The word was they were left over from a South or Central American contract......probably BS.
I bought one of each for about $700 each and transferred them through Crawford International in Silver Springs, MD back in the day. When the guns arrived there and I was taking the paperwork to the Maryland State police for fingerprints and sign off.....Ken Crawford the owner said that the guns looked like over runs from the contract for El Salvador.
My guns were the same as yours except mine had 1 in 12 twist barrels and the front sight towers had a small bell forged into the side. Stocks were the 1970's vintage plastic dipped alloy stocks and not the lighter fiberlight stocks that your had. The Commando barrels did not have the bayonet lugs, and they were roll marked "C MP CB" underneath the barrel and not on top.
The important thing is your piece has the remnants of blue paint on the stake marks on the receiver extension and bolt hold open.....which usually shows an original carbine or shorty.
Mike Lester, of Class 3 Supply, when they were in Nashville, TN had several M16A1 Colts with 14.5 and 11.5 inch barrels......and I think all of his had the bayonet lugs.......going with your theme that Colts used everything they had in the parts bins........
The late Bob Roy of Colts Engineering and International Military sales explained to me that for export sales, Colt would use up whatever parts were available to meet sales requirements, versus commercial (Police or Prision) and US Military sales (which required everything to be MILSPEC).
When I did a PSD in Panama and El Salvador in 1985 for the US Navy, I carried a Mini Uzi and a Colt M16A1 with a 11 1/2 inch barrel exactly like yours with the bayonet lug.......it came out of the US Embassy cache for security types and it was still brand new when I sighted it in at the Para base in El Sal.
Nice find on a great condition A1 Commando.......
Your Helmet Will Not Protect You If You Are a Dumb Ass!