I have never seen one like that. That appears to be a modification to me, but why was the cut made so low? It looks like either an extra fuel tank or maybe a diesel tank to add fuel to equipment. Or some other device, too hard to tell from the pic. When I enlarge the pic, there is lettering on the door, its not legible, but it makes me wonder, if it was Govt surplus. I can vaguely see what looks maybe like, US Department. The Green and white paint job leaves out military. Maybe US Forest Service, or National Parks. I'm just guessing from one pic.