The keyhole means that the raw, solid forging was sourced from Cerro Metal Forgings.  Nearly all AR companies have used Cerro forgings, and Cerro doesn't do any of the machining or finishing work on the forgings.  So, effectively, the forge die markings you see on upper receivers are pretty meaningless when it comes to "who manufactured this upper."  And since uppers aren't, as a rule, marked by the manufacturer, there's no way to tell.

-Troy