The nut for this bolt is the same as the nuts on the bottom of the trailer hitch (if you have the tow package). The bolts that connect the stock rear bumper to the hitch. The top of the bolt is covered by the plastic cap on the bumper.
Last year at a meet we noticed someone missing that same nut. We were able to remove the nut from the hitch receiver and use it since we were not towing anything.