Haha, thanks Mike! I'd just finished composing a lengthy response to Colin's question, speculating on the purpose of the sunroof, and when I went to post it I saw your reply! I'll post it anyway as it discusses the canvas door as well.
It's an excellent question you raise Colin. As a purely Australian innovation, the use of a sunroof is commonly, and not unnaturally, explained as a climate adaptation. However this makes no sense, because as Keith points out, it was used exclusively on the 15 cwt. Furthermore, the canvas doors introduced on all late production Australian CMPs, which are also commonly explained as a climate adaptation, were never used on the 15 cwt.
Therefore it becomes clear that these two Australian innovations are related primarily to vehicle function, and only secondarily, if at all, to driver comfort. Hence we need to look to vehicle role, rather than climate, for an explanation of their purpose.
To my mind the clue lies in the canvas door, which is specifically designed to be removable, as evidenced not only by the hinges, but also by the accompanying strap across the doorway, to prevent the driver falling out of the cab with the door removed, and the accompanying metal strip along the floor edge, to prevent articles sliding or rolling out of the cab. All of which tells us these are not canvas doors per se, they are removable doors which just happen to be canvas covered. Just like the canvas covered sidescreens, their principal purpose is removability.
The question then becomes, why the need for removable doors? The clue here may lie in the fact that late production Australian CMPs were almost exclusively 60S specialist vehicles, plus the aforementioned 15 cwt vehicles which never received removable doors. The majority of these 60S specialist vehicles appear to have been tippers, plus a few water tankers, the operation of which would require the driver to be constantly in and out of the cab. For example, tippers engaged in airfield construction in the islands, ferrying crushed coral back and forth all day, and doing lots of reversing. If you've ever reversed a blitz on hot days you'll know that the door must be opened, unless you want to burn your right underarm on hot steel! Once opened it invariably swings wide open, which is not only a nuisance but can also result in destruction when you reverse past a fixed object! Hence while passenger doors are plentiful today, driver's doors are rare as hen's teeth - they've all been removed in commercial use for this very reason, either prior to or after destruction!
Conversely, the vast majority of early production CMPs in Australia were 60L troop transports, intended for use over some distance without constantly alighting the cab, and generally driven in the forward direction, even when retreating! As such they have no need of removable doors, and apart from these 60L CMPs we see mostly 15 cwts, with very few early production 60S vehicles. Of these 15 cwts only a small percentage are fitted with sunroofs, and if like the removable door we look upon them as removable roofs, rather than canvas roofs, we may discern their purpose.
Is it possible they were intended to replace the earlier commercial bodied pickups which had undergone rag top conversion? These are commonly seen in arty units, along with the rag top LP3 gun tractors, and while I'm not sure of their intended purpose, I imagine they'd be useful in artillery spotting work, or general reconnaissance work in gun siting. Apart from that I can only think of parade work, for the brass to ride in standing! Mind you they did seem to have more than the usual frequency of parades in arty units, what with demo shoots and the like, with brass in attendance.
I have an idea the sunroof included provision for a sun compass, which if true would lend weight to the reconnaissance theory. Anyway that's all I can think of myself, and I notice they do seem to have been quite common in arty units, like the 2/7 Fd. Regt. vehicles seen below.
