Here is a picture of a restored 1940 Ford Open Cab Pick-Up. Notice on this one, that the Pick-Up Bed is totally separate to the Body.
I do agree that the illustrations from the original Sales Brochures look like the Pick-Up Body is part of the Cab, but as you can see on this example, they are indeed separate. I am still trying to find pictures of an original Closed Cab Pick-Up so we can confirm one way or the other.
However, it seems that very few of this model have survived? I will have to continue my search. If anybody elas has anything, can they please add their comments.