Concerning the car's history, given my memory, the plates on the car were most likely issued in late 1992. It is not clear when the car was imported into the US or whether it was registered in any other states prior to California but the period between 1974 and 1992 would be evidenced by other, older plate(s). My impression is that the car was a German market car that I would expect it to have been "Federalized" when it cleared customs, so a paper trail of some sort would be out there somewhere. Did your Attorney have any of the paperwork?
If the car has been on Non-Operation with DMV since 1993, and you are interested in learning more of your car's history here in the Golden State, there is a possibility that the California DMV paperwork from its original registration here may still be on file. If so, one of the things that would be part of the file would be the Vehicle Verification form that includes a place for the prior license plate number and the odometer reading. A bit of detective work might yield some interesting pieces of the puzzle. If you decide to dig in that area, please post any results for us to see.
Good luck with your project.