does anybody have or know the letter codes on
upper receiver's. the one i was looking at i
think it had a "F" by the rear sight.

Thanks,
Gary

Generally speaking, the only markings on uppers are the ID mark(s) of the forge that turnrd out the raw blank, and since all the manufacturers are using the same forges these days, you really can't tell who actually made the upper.

The one exception is Colt, which historically had the forges put a "C" along side the forge letter under the rear sight.  More recently, when Colt has used a forge that other makers also use (Anchor-Harvey), I've seen "Colt" marked on the upper.

Apart from that tho, it's anybody's guess who the manufacturer is.