1968 Gmc ground up restoration

[AK72CHEVY]

Sorry for bringing up a quiet thread, but just read it for the first time this morning, and all I can say is WOW. Great read for anyone that has missed this. Couple of things I was curious about was the purpose of the extendable rear bumper and the plug above the driver side wheel well on the first page. I didn't notice any mounting holes for a fifth wheel hitch, which I would assume was the intent for the plug. I could see myself using a bumper setup like that for supporting and hauling long pieces of lumber, but not sure if that is the original intent. Anyone have any ideas? Anyway congrats and thanks to the op for such a beautiful truck and build thread.