There is nothing I am aware of attached to the cab itself. After some models of truck production switched to tanks in the cab, the area (early ‘49 running change) in the corner was left open, uncovered. Perhaps he is thinking of the early cabs which were enclosed in these metal panels?
Early cab corner filler panels.
https://www.flickr.com/photos/advance-design_parts_co/15724662629/in/album-72157649468726416/
https://www.flickr.com/photos/advance-design_parts_co/17058544799/in/album-72157651718017649/
Early ‘49 and on cab showing tank relocation, panels removed.
https://www.flickr.com/photos/advance-design_parts_co/11974011234/in/album-72157639819091033/