I picked up this pair yesterday with intentions of using the 20" barrel to build a retro gun for my father . I really dont know much about Colts as far as dating and codes but I know they have a stamping near the FSP as a production date .
The 16" is for sure a HBAR  , the 16" has a date code of 3/96 the 20" does not have a date, it just has a 15 stamped in the same area in which the date code would be ,which leaves me stumped for a production date , The ramps are M4 ramps as I put it in a mil spec upper and everything matched great . Neither have the bayonet lug on them  and the 16 has a funny looking compensator on the end of it. The 16" had a Colt alcoa forged upper and carry handle .  Also  Any Colt guys out there have any info for me on the 20" barrel with the number 15 stamping as far as production, barrel model  etc ?
Thanks , Heres a few pics
16" HBAR  carbine












20" rifle length

the 16'' comes from a 6731, the muzzle device has been added later by threading the barrel.

I guess the 20'' comes from a R6551.
According to the USGI M16 variation guide Colt started putting a 2 digit lot code by the FSB in 1989 and the mfg date in 1995 , so your barrel must have been manufactured between '89 and '95.

Thanks Frens , big help to me , thanks for pointing me in the right direction

Frens,
Any idea what they would have for value ? Im keeping the 20" for sure for my dads build and I will probably be unloading the 16" to help pay the way for this build.
Thanks
PH