Maybe you have a splash apron from a Model A that had a side mounted spare tire. If so, that hole is for the tire support arm that is bolted to the frame and extends through the splash apron. A triangular plate bolts to that and the spare tire with wheel bolts to that via three lug nuts. Perhaps your car was put together from parts lying around the garage or a past replace a damaged side splash apron? A photo of the hole and surrounding area would be helpful to make sure our answers are on the right track.
Marshall