Thanks for the close up. IMHO the currently unused hole appears to have some additional plate behind the frame rail and we can see it has been used to mount, most likely, a transverse muffler at some point in the past. There is allot of metal in your picture that shows a thread pattern like those produced by a self tapping screw as used for that purpose at the plants. We also see the deformity (inward) in the metal around currently unused hole showing the focus used to start the screw that was once present. Also (red arrow) that could be a spot weld past the second hole that your using for something else not originally there. Does appear that the drivers side frame rail is original given the stamping date of what looks to be May
I have also included two examples from cars that were not equipped with a transverse muffler system showing the thin single layer we see on those cars for comparison
Next place to check is the inner surface of the drivers side frame rail in front of the area where the tie down brackets would have been attached. Out to where it bends upward following the front of the trunk floor. Your looking for two holes close to one another, how they are orientated, if they have been used and any other details