There's another possible aspect to the "Broadcast Copy under the rear seat story" , but not on first-gen NOR cars. Norwood was an old Fisher/Chevrolet plant, and followed the same overall sequence as all the other Fisher/Chevrolet plants, where Fisher supplied a painted and trimmed firewall-back body shell to Chevrolet and Chevrolet made a car out of it.
GMAD plants, however, weren't constrained by Fisher traditions, and used the "soft trim after Final" sequence, where all of the interior soft trim parts were installed AFTER roll-test and water-test as a driveable car (Corvette also followed that sequence); easier to spot water leaks and no water damage to costly interior parts. At some point after GMAD absorbed Norwood (don't recall when offhand), Norwood changed to the GMAD sequence, and that system would have made it possible to find a Broadcast Sheet under the rear seat.