Palmetto Armory and Palmetto Mfg. are two different companies.
Uppers and lowers are forged by outside contractors and are
machined by whoever builds them into guns.
For example:
The CM on Colt uppers is, Colt-Martin Marrittea
The CH is, Colt-Harvey Aluminum Co.
I read a tech manual a long time ago and found this out.
Also check out www.biggerhammer.net