Looks like a piece of someone's leaf off their spring. I was going home one day years ago when I still had my Super Duty, I was almost to my exit on the freeway. I see this truck in the left lane coming up fast on me, as he passes me I watch his ball mount on the trailer hitch fall out. Hits the road, digging up stone as it skips along behind him, headed for the median. Luckily it went far enough that it wasn't a issue any more, but had someone been behind him, could have been a different story.