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People often say "FN" when they don't know the origin, or if it's obscure. FN doesn't sell M16 parts outside of military contracts. That doesn't mean the uper is bad; just that it won't have a warranty, or any sort of guarantee.
Pretty much the only way to identify make of an upper is to check the barrel, and even then, that only indicates the barrel's manufacturer. Without marks, it's impossible to determine.
Upper markings simply indicate the forge; they don't typically narrow down the maker to anything more specific than two or three manufacturers.
My guess is that you got an upper made by someone in their garage with pretty generic parts, which is pretty much par for the course for gunshow uppers. The rear sight is relatively hard to put together, so someone may have incorrectly assembled it. I suggest you put it together; after that, I'd look at an overtorqued barrel, if you have windage issues.
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how much you want for your gunsmith opinioned "junk" upper?
2-5 tops mfg of the actual upper. any ar can be fixed, unless there is excessive headspace (closes on a field guage) the bbl has been shot to sh^&*T or bent.
as for the rear sight you need to knock the cross pin out (keep an eye on the spring) take it off check the ball detent & spring in the l/side of the sight. it might have a spring that's too long putting pressure on one side cocking it.or to short allowing it to flop around. when you reassemble you need to compress the spring inside the hole of the rear sight so you can get the pin back in. i dremeled a notch into a crappy screw driver for that custom gunsmith tool
