Interesting I wonder if they are just older panels before they were making the ones with stamping on the side or if someone was able to bend the angle from a 45 degree to a 90 degree to match the 54 .could you measure the depth of the box from the 90 degree bend to the angled bed strip the 54 had a deeper box than the older trucks .That way you could be certain .The frames are apparently different on the 54 to allow for the deeper box design but I am not sure of this ,I do know the 54 does not have the wooden bed blocks to hold the bed up and that may be the reason to hold the older bed up higher and not a difference in frames .Maybe check in the tec tip area were it shows the differences between the years
