Understood.
If the press preview heritage can be verified, then is is possible it was pulled from line and filled in during body in white and completed after?
Even if filled with plastic/lead it would be interesting to verify. Pull the back seat panels and have a look in there. May be able to tell if originally there, and refinished during resto.
I find it curious it's not cracked and fractured. That era b body was prone to cracking in that area especially if the car was put through it's paces.