Boy, has this thread moved off it's intended course! anyway, the hole in the rear cross sill is indeed for the J hook of the spare tire carrier on some trucks. My '48 3100 uses this hole for that purpose. Other trucks have a different setup and do not use the hole for anything. I'm guessing the cross sill was manufactured with the hole, and they just left it that way when they used a different carrier.
Now, back to the original question. My thought is that the center bumper bolt was just aesthetic. Perhaps GM had an idea in mind for the center hole and the plan never came to fruition.
