[Wavery]

I'm going to throw out a guess here.

With the advent of the back-up camera, that bolt may be to prevent the coupler from accidently closing while backing the tow vehicle.


On the underside of the coupler, there is a "shoe" that grabs the underside of the ball. On most couplers, that tongue has an adjustment nut. That nut can be tightened (slightly) so that the "Tongue" will grasp the bottom of the ball with a very small amount of play. If you tighten it too much, the coupler may not close all the way or close so tight that it will wear on the ball. It should just barely touch the bottom of the ball with the coupler closed.