You are missing the detail of construction and attachment. The boards
are not screwed down to the bed, but are put inside a system of steel
battens that run the length of the boards and bolt the edges to the
frame.

Either Ford, Chevy, Dodge, IH or maybe even someone else used plain
square edged boards. One or two of the previous used rabbited boards
to fit inside the steel battens.

Having said that to T&D, could you contact one of the many amatuer
restorers at the car shows in your area. I used to know a guy here
that restored Hudson trucks (that's right... Hudsons... he has 3!) and
he was the picture of imagination to get the info and parts he wanted.
He went to antique/classic car shows to take pictures and measure
stuff.

Robert