At one time Colt made their own receivers in house, there were a few companies that supplied them the raw forging but Colt finished them out.
Nowadays I've seen the S marked triggers in enough LPK's to know they are outsourced. But as far as who does what for who, I don't think anyone can tell you that with any reliability.
I did have a conversation with a guy that inspected outsourced small parts for Colt. His take was it's not who made them but the level of QC that mattered. He said when he rejected a shipment from a company, they were just sold to another company with lower specs. So while the same company might supply parts for others, not all parts were the same quality.
