Hi All
I'm not sure that clear starting date can be drawn, I suggest this because in reviewing the many original photos posted over the years on MLU you see large numbers of Pattern 12 cabs with the square hatch. I think that some of the lack of a definitive statement also comes from differences in introduction date between FORD and CHEVROLET then add in Canadian vs Australian production. Also there is the possibility that the early square hatches were retrofitted to earlier Pattern 12 and 13 trucks which had originally been manufactured without them but shipped as knocked units for assembly.
The HUP family of CMPs seem to have hatches from beginning to end.
Neither my `41 Pattern 12 C60L or `42 Pattern 13 C60S have hatches while my `45 HUP of course has the round hatch.
Ok, who has got the production notes/logs to sort this question out?
Now back to Howards question concerning North Africa and the introduction, of observer openings in the roofs of CMPs. I'd suggest doing a search on the
http://www.awm.gov.au/ if you have not already done so. Lots of interesting photos. I do remember being told or reading that the origin of the observer opening was in response to the issue of not being able to hear aircraft approaching when ridding in a CMP. I have noted that in a lot of photos of CMPs on the move in the Desert there is somebody standing in the back of the truck behind the Cab who I suspect would indicate the approach of aircraft to the driver by pounding on the roof of the cab.
Cheers Phil