> 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?

That's a temporary operating permit, colloquially called a moving permit. I've not built a car, so I dunno what happens there... but I've gotten moving permits for cars that have expired registration and need an emissions test to get registered. Can't have wheels on the public road unless it's registered or you have a movimg permit.

Afaik, all of our states have a process to get a VIN for a car without one... although it's common to take a VIN from a similar donor vehicle because it's less effort and vehicle safety and emissions requirements are de facto tied to the model year on the title. Legally, they're tied to the year of manufacture --- emissions ties to the year of the engine, not sure about safety.

You see all sorts of shenanigans if you look though. VW ended sales of Beetles in the US by the early 80s but still manufactured and sold them in Mexico through the 90s... it was not uncommon to send your US titled beetle to Mexico for "repair" and get back something very different. Sometimes just an engine transplant, sometimes only some key component from the US vehicle like the frame or the floor pan would remain, sometimes just the VIN plate. Putting a newer engine legally triggers newer emissions standards, but if you don't tell anybody and nobody notices, you can get away with it for a long time.