It would be nice to have the receiver/barrel out of the stock so we could see the shadow at the point of assembly barrel/receiver and the side of the receiver where they stopped sandblasting and the area there. It looks like in that one photo you see what looks to be the area of interest.The parts swap wouldn't concern me in the slightest it could be a field modification for a good reason. I'd love to have a six pound trigger on any carbine I have.
It is a nice gun.