Since the truck line shared a lot with the passenger car line, did the station wagons of the past have this type of set-up?
They look like 3/8" fine thread. A type of slot bolt. Could easily be fabricated.
On the Chevy's do they use a 3/8" or 7/16" carriage bolt with a wingnut?
Woodcraft
Toolless seat adjustment
Amazon.com
3/8-16 thd. (A), 1 1/2 (B), 1" (C), 3/16 (D), 11/16 (E), Table slot - 3/8, Standard Type, T-Slot Bol
by Reid Supply Company