I believe it is a carryover from 39 when the spare tire was mounted on a stand up casting mounted to the running board. Cost reduced in 40 to the stamped bracket that was riveted to the bed side ( which almost all failed at the rivet and farmer welded back on). The hole was left in.
Just my opinion.
Whats the hole for?
Do you still have your pickup or are you selling excess parts?
Lonnie