1937 was the first year for mounting the spare tire under the bed. Prior to that time the spare was mounted in front fender wells on the passenger side.
The under hung spare tire mounts for the early trucks are as rare as chiken lips. The only one I have seen is the one I have. The device has a two parts. I have seen several of the bed mount parts, but the underhung part that holds the tire up under the bed is tough to find.
