This is a continuous topic of discussion.
Two things to remember:
#1 There are companies out there that produce raw receiver forgings and sell them as raw forgings to firearm manufacturers.
#2 These raw forgings are purchased and finished by the manufacturers and assembled on their product (rifles) and sold.
Unless you have a "C AF, C M, C MB, LMT, Les Baer(?)or some other upper with definite manufacturer/finisher ID like that, there is no real guarantee of who finished the receiver unless you have received it directly from a manufacturer.
I've had factory Bushmasters and DPMS with the keyhole mark. Supposedly even some Colts have this mark, although I have not seen one in person yet (I think I saw a picture here once). I also have two factory Bushmaster rifles with the broken "A" - Anchor Harvey forging mark. Probably comes down to which forging company has the cheapest/most available raw forgings when the need arises by which manufacturering company needs them at that time. Bushiness is business.
So a statement such as "That upper is Bushmaster because it has the keyhole forging mark" or some sililiar statement may not always be correct.