I got a Koenig extender box from a jeep in the 40's. it was petty rough but take the dimensions of it , constructed just the way they did ,over built, It being made with 16 gage metal , the bottom has a cross x support and
1"x1" angle support the sides ... the extender box would go on the Jeep,then the tailgate would go on the extenderbox with two rods from the hitch to the end of the box near the tailgate. I had one on my jeep and remember bring home a 390 cu in engine in it. pretty sturdy. I built 10 of them and sold them on ebay and to jeep people . I put a price on them to low .at the time . Is there any interest in the extenderbox? i have a sheetmetal shop and looking for projects.