Mr. Balance has stated that about 4600 receivers were made while he owned the company in Devine, TX. Serial numbers under about 03800 should show stamping on the underside of the barrel between the gas cylinder and the bayonet lug with an address including Devine, TX.
Some receivers were included when the company was sold to Springfield Armory and were built into rifles by them.
These are not marked Devine, TX. These could be marked Genesco, IL.
If the serial number is after 04600 it is a Springfield Armory made receiver.
As far as GI parts a four digit rifle should have all GI parts. Just write down the alphas you see on the bolt, op rod, trigger group. They should look like WRA, HR or HR-something, TRW, SA. We can help identify them for you.