The FAM has a couple diagrams of the side mount spare. RPO 341. Section 10, Sheet 3.00 and 3.01 But the FAM being '54, it shows the flat top bedsides. Angle brackets bolted to the bed side and rods bolted to it and the wheel plate.
I think lots of people invented their own style, which is probably what's pictured above. Or maybe there were aftermarket suppliers back then.
