Hello Mike and Welcome Aboard!. Always glad to see new bolters join in the fun. Hope I can be of some help on your questions.
A. The dump mechanism is and has always been an aftermarket upfit. Likely the body too although some platform/stakebodies were factory installed. By that I mean they were ordered by a dealer direct from GM or FoMoCo and perhaps others. These bodies were produced by other builders and shipped to Detroit where they were installed by upfitters and then shipped to the selling dealer complete. A variation of this, the Chassis Pool upfitter, is still around today.
B. I'll let someone more familiar with the 46' help on this.
C. The hole was for the hand start crank and there was no cover to my knowledge.
