[Alex van de Wetering]

Richard,

I recognized the square holes with all the nuts around it, that the previous owner cleverly used as windows for the cab rear wall.....they are the original frames for the access covers (vents??) in the sloping back.
The best I can come up with at the moment is this factory picture that shows the two covers.
source: factoryphotos DVD from Servicepub

I presume the square tubing is also part of the original super structure.