Ken - I read through Chevrolet Shop Manuals for 1938, 1940, 1941 and 1942 and none show there being a hole in the spot on the truck gear box cases. In someones mind
perhaps, it might have made sense to put a hole there to allow for air to escape when filling the gear box through the threaded port above that hole, but then leaving it open to allow contaminants inside is counter productive.
I think you can safely plug the hole and move along with the rebuild of the unit.
