It's the markings of the aluminum company that provided the raw forging.  Virtually all AR companies buy receiver forgings from whichever aluminum company is making them at the time.  The forging itself is identical; it's the machining and finishing that matters, and can vary from manufacturer to manufacturer.

As you probably know, some manufacturers also use cast receivers, which are less expensive because they require much less machining (more detail can be added to a casting), but cast parts are not nearly as strong as forged parts, especially when it comes to impact resistance.

Anyway, upper receivers aren't marked by the manufacturer, so if you don't buy direct from the factory, there's no way to know what "brand" it is.  (There are a couple of exceptions to this rule; for example, Colt M4 receivers have the "4" or "M4" stamp above the gas-tube hole, and older complete Colt rifles would have a proof-stamp on the upper receiver.)

-Troy