Could any of you hard core Retro guys sort this out for me.  I've had this assembled barreled upper since 1992.  It shoots just fine.  I got it in a trade.  For some reason I've recently decided I would like to know just what those barrel markings mean.  I've already found some old threads discussing similar barrels, and it seems FNMI likely refers to barrels made by FN in the past, and that 4T663 is a CAGE code for an outfit in Florida that used to make replacement barrels.  I know the CM coded upper receivers are Martin Marietta contracted for Colt.  I'm guessing this is simply a replacement barrel on a GI receiver.  The problem is I can't find anything that settles the apparent disconnect between the CAGE code and the FNMI.  Would anyone care to comment?

Yes, it is for sure a replacement barrel. It sounds like FN had the contact and subbed it out to Engineering Research.

Well see, that was my guess too, but I wondered why FN would subcontract barrels.  Maybe they needed to meet a deadline for delivery and had to get some help?

Found the following from an archived post from a very in the know person. He also mentioned that Colt would look to subcontractors when they where short on parts to fill a contract.

“4T663 is Engineering Research Inc. They were in business from '82 to '89. Their barrels are usually found on rebuilds when they are encountered.”

Thank you!