Agreed and with all of that I wouldn't worry about a tag or build sheet. Now, build sheets are quite elusive, most cars had one, some of those didn't match the car, and some cars didn't get one. Very Very few California built cars had a build sheet unless it was a factory exec ordered car. They had IBM punch cards for their assembly process. The OP's build sheet could still be in the car (or a second sheet) on top of the glove box liner, under the bucket seat backs, under the carpet, or under the headliner. Thay have shown up different places other than under the rear seat back springs. Your build sheet could have gotten put in the next car down the assembly line. Sometimes they get turned in to places like "the Hamtramck Registry Library" or even to Galen Govier "Galen' s Tag Service" (GTS) .