There's no way to know, because it isn't a manufacturer's marking.  It is the marking of the forging house that made the solid aluminum "raw forging".  AR manufacturers buy these raw forgings from whomever is making them at the time, and it isn't at all uncommon for the same markings to appear on many different brands of uppers.

The manufacturer is the one who takes the raw forging and machines it to final specs, then has the part finished (anodized, in this case).  Since upper receivers aren't usually marked by the manufacturer, there's no way to know who the manufacturer was.

-Troy