I can not tell you for sure but I do believe that is for a 2 speed rear end. I know my 75 D-800 had it. That bare fuse block was used on all D & W series trucks. So maybe someone swapped it out before you got it. The one other item that comes to mind on trucks with the camper special package. It is possible that it was used for a camper harness. For now if you are not concerned about being factory original just make a diagram and remove it for now.