Another big point of curiosity for me is registration--it clearly doesn't have a CA number plate, but there was a big red "3" on a laminated card in the spare.. what is that?
How do you go about titling and registering a brand new body + frame?
Not OP, but since I looked up the corresponding UK process: https://www.gov.uk/vehicle-approval/individual-vehicle-appro...
You end up with a special "Q" (as in James Bond?) number plate. There's a whole bunch of different processes depending on how Car of Theseus the vehicle is. Apparently this has recently been streamlined for rebuilds of classics from original parts (which is _not_ what the Jeep is). The system is still intended to prevent the re-use of crashed or otherwise written off frames, though.
That was also a fun story! I had to do the insurance for it and CA is actually surprisingly a great state for this kind of stuff.
Typically in the US you get a donor firewall from a registerable vehicle that has the VIN plate. This is pretty common for things like civilian jeeps, chevrolet C10 pickups and a few other American cars/trucks that have complete bodies you can buy in the aftermarket.
For military equipment you usually need the original frame (or part of it with stamps) because it has serial numbers stamped on it for the DMV to go by when registering.