What is the keyhole marking on upper receivers?  Is it a manufacturer mark?

It is the mark of the factory that made the raw receiver forging.  The keyhole is Cerro Forge.

+1


Ref: Receiver Markings thread link

Well, that was informative.  The only marking on my Olympic 45acp upper is a  "K" which would be Kaiser Aluminum.  Thanks for the info!

WKM

FYI Cerro's website link.


www.cerrofabricated.com/

I have an upper with the keyhole marking and nothing else.  Did cerro forge and machine the upper too?  Or do they sell the raw forgings to colt, bushmaster, rra etc.?  Basically how does one tell what company the upper comes from?  When I bought the upper I have I was under the assumption that it was Rock River, but there is nothing to indicate this.  I have another AR15 with an A2 upper having no identification marks at all.  I only inquire as I am in the process of building another AR and would like to keep uppers/ lowers from same company whether it be Bushmaster or RRA or whatever.  

Cerro only produced the raw forging.  It was then machined by someone else.  It's almost impossible to tell the brand of an upper receiver.

Thanks all.....