[Tony Wheeler]

Thanks Mike, it all makes perfect sense to me now. The canvas doors were a climate adaptation, and any other advantage they may have conferred, such as ease of cab access as I speculated, would have been purely incidental. I guess the precedent was there in the Cab 12 door, designed to be fixed partially open for increased ventilation, but obviously not transferrable to the forward hinged Cab 13 door.

In relation to the 15 cwt though I think the point about vehicle role remains valid, in that it wasn't considered necessary to provide increased ventilation. That makes sense to me as it really was little more than a base tender, small wheeled and low geared, and the uses to which it was put in artillery units, including designated gun tractor on occasion, were well outside its expected role. As such it would logically be best catered for by retrofit where necessary, which seems to have been the view at the time. I didn't know such a kit was available, and it may explain the one and only F15A I've seen with canvas doors.

I'm still curious about the timing of their introduction, which was simultaneous with the introduction of 6" steering ends and 400 steering box. I'm yet to encounter a single early production chassis with a canvas door cab, or a single late production chassis with a steel door cab, except of course the 15 cwt. And yet the two developments are functionally unrelated, and they even arose in different countries. Which leads me to wonder if there was a break in production between the early and late chassis, during which time the '44 cab was tooled up and ready to go upon resumption. I just can't see how they could coincide so closely otherwise, seemingly on the same day.