The steel panel with oval cutouts directly behind the cab actually had a different purpose than the one that it is currently fulfilling: it fit directly behind the rear window on a full-height stake bed and allowed better visibility to the rear.

The elbows on the horizontal wood runners appear to be custom. Although pre-AD trucks factory-equipped with flatbeds often had steel corners that looked like quarter oil barrels:

http://www.brads41-46chevys.com/id24.html

You will find newer trucks with the ribbed bedrails (into the 1970s). Talking the owners out of them will most likely be a challenge.

Good luck.

- AJ