You're correct on the square upper receiver marking.
https://www.ar15.com/forums/ar-15/Square_forge_mark_on_upper_/118-437739/ Not a ton of information in this post but maybe it'll help some.
The Troy rear sight is great - actually my favorite BUIS set personally, especially with the HK-style front sight. TBH I think that looks like a mid-length gas, not a rifle length gas system. More than likely it's a PSA, just by economy of scale.
https://palmettostatearmory.com/psa...r-with-bcg-ch-mbus-rear-olive-drab-green.html But tbh? Bro gave you a free upper. I'd stick with the spirit of the gift and either run it as an acceptable personal rifle, or give it to someone you care about who needs a gun. If it runs well on one of your existing lowers, I'd plop a good-but-cheap red dot like a Sig Romeo 5 or Holosun 503R on it and attach a Vickers BFG sling to it and run it.
For BCG, Toolcraft or Microbest would be the two I would recommend for it. Charging handles, I'd either do a basic milspec if you need to keep it cheap, or either a Radian Raptor, BCM, or Geissele Airborne if you want to spend more money but get a more handy product. Don't buy a cheap Chinesium knockoff CH though, if you can help it.