I bought this upper at a gun show several years ago.
It is the top one in the picture.
It is a COLT with what looks like to me an AK flash hider. It is welded on.
Barrel marking  C MP 5.56 NATO 1/7
The upper is marked C MB
Ok, I looked at it closer, Maybe it is  O MP 5.56 NATO 1/7
STILL NOT REAL SURE...........
I have seen a couple of them over the years, but I don't have any information on them.

Any help.

thanks
ron

 

It's a ban era Colt lightweight Gov't carbine . In those years they were not allowed to sell AR's with flashhiders , bayo lugs and collapsible stocks. Your carbine had no flashhider so someone installed a AK-74 style muzzle brake( which was legal during those years ) on the factory Colt  1/7 light barrel. It would have come with a fixed A2 stock as well which has been swapped for a non-colt collapsible. Those mods are now legal with the sunsetting of the AWB . I would ditch the AK brake and get an A2 hider installed. The AK brake works great but are loud as hell and bright at night on an AR. Hope this helps, Michael