There aren't any.  No AR manufacturers regularly mark their upper receivers.  Colt used to put proof stamps on all complete rifles, including marking the uppers, but their seperate uppers and parts were not marked.

The markings you see are those of the company that supplied the raw forgings to whomever actually machined and finished the receiver.  Pretty much everyone gets their raw forgings from the same sources, which change all the time, so it is normal for 4-5 companies to be using forgings from a given supplier.

-Troy