Bushmaster lower serials:
"L" prefix is manufactured by LMT
"C" prefix is manufactured by CMT
"BFI" prefix is made in-house
There are earlier versions which were pictured above, but this will get you through the 90's to current for sure.
You would have to ask Mack Gwynn who he bought the first receivers from when he started bushmaster way back-in-the-day.
Mack's current company is
MGI