US&S pistols were only made during 1943. Colt was the only manufacturer to overlap serial numbers with those of the contractors, and only in early 1943 during the initial confusion over four companies making M1911A1s all at once.
Yes, the frame is apparently Remington Rand and the slide is off a Union Switch & Signal. Military mixmasters are fairly common. My dad for example has an M1 Garand with a Winchester receiver and Springfield Armory barrel & small parts. My M1 Carbine is an Inland receiver and barrel, with a Winchester stock & Underwood front sight. The 1911s in military inventory didn't escape any of this parts switching either. When lots of small arms went in for maintenance, they were all stripped and the barrels went in one bucket, the slides in another, and so on.
The chances of finding a rifle with all its original parts is very remote, thus the vast majority of "correct" rifles are in fact "restored". The reason for this is the vast rebuilding programs in the post-war period for M1 Garands and Carbines. There was no rebuilding program for 1911s however, so a much larger percentage of original, correct specimens exist. In addition, a lot of the minty 1911s you see were in fact smuggled home after war's end so they stayed 100% original. It was pretty hard to break down a Garand and hide it inside your duffel bag!
Me too. The recoil spring fits on the guide first, then is dropped into place in the slide. The hardest part of reassembly is getting the barrel link lined up with the slide stop pin hole and re-inserting the slide stop. I can do it in my sleep nowadays, but way back when I was still in diapers I had trouble getting the slide stop back in.