My 49' underbed carrier slides into an angled channel that runs from front to back centered at the rear of the truck. I actually have this on two trucks. It is held by a j-bolt hook at the rear to the bed cross bar for easy access. The later carrier (post 49') switched to a bar that runs across left to right and is hung by large bolts on each side of the frame behind the rear axle.
Maybe that is what you bought.
