On the 1621HD, you can't look down the inside of the pin box from the back like that. There is a metal plate towards the bottom, much like the one you can see in your photo other than it goes from top to bottom, side to side of the whole pin box frame interior. From what I could see of the internal brace with an inspection camera it looks just like the U brace in the photo, which I suspect is welded to the aft plate in the pin box, but I can't verify. There is also a V piece of steel welded into the aft portion of the box that runs from where the pin box bolts to the trailer frame to the front plate in the pin box. The weld that you can see in the photo I posted of my pin box where the crappy weld goes side to side is where the plate filling the interior of the pin box is.
As for why the pin box got bent it seems pretty simple. In using the Andersen ultimate hitch adapter I put loads on the pin box that it wasn't designed to handle.
