At last research, DPMS and Spike's lowers are forged by LAR Manufacturing. S&W by Continental Machine Tool... and PSA by Olympic. This all may have changed at this date.
The actual manufactures listed also make lowers and uppers for a number of other companies. as well, in some cases for their own retail needs. Specification for the individual end users will vary based on features and the final finish process... and who actually does the final finish.
A good example of this is that my information shows CMT to be the maker of lowers for:
Rock River Arms
Century (New supplier)
CLE (Compass Lake Eng)
MGI (Hydra Modular System)
Grenadier Precision - Now LWRC.
A bit of price and quality spread eh?
Shoulder Drive Nicholson Club