In the interest of full disclosure:
I was the one that boxed the frame. I looked at my "day 2" pictures, and I did it to myself, about 5 years ago. It's no excuse that the bumper wasn't mounted when I bought the truck. My clue was, when I looked at the welds a few minutes ago, they're only on the front of the nut. If they would have been welded on before boxing they would almost certainly been welded on two sides. At least they'll be easy to cut off.
Thanks much to idbeast. Now you can all laugh hysterically.