Keith, sorry mate, I mean the CGT-based 2-pounder Portees in the photo I posted...16" or 20", as I am no expert.
I had a check of all known 1941 Model Chevrolet CMPs and suggest that the change-over is most likely to have been in amongst C-GT assembly, going by known sequential production numbers.
As to the change-over of cabs, I discussed this in another thread recently:
Changeover Thread
>THUMB SCREW TYPE ENGINE HOODS…BEFORE SERIAL # 084XX04001 AND # 184XX05004 (# 11 CABS) then
ALLIGATOR TYPE HOOD..ON OR AFTER # 084XX04001 AND 184XX05004 TO SERIAL # 284XX01523….(# 12 CABS)
[ also SOLID WINDSCREEN then ADJUSTABLE WINDSCREEN]
I have mentioned before that both Ford and GM of Canada continued 1940 Model production into 1941 to distinguish the 1940 contracts. British contracts placed in late June 1940 went into production arguably before 1941 Model Year had started but was parallel with the 1940 contracts. We also know that 1941 Model production continued after 1942 Model Year started, but for 1943 and 1944 Model Years this arrangement had ceased, though there were no 1945 Models, and all production to the end were 1944 Models! As to when the changeover occurred this must have been sometime in early 1941:
C15A serial # 0844404056, engine # TBA2,816,647 was a # 12 Cab, and 0844403851 engine # TBA2,816,584 was a # 11 cab!
The nearest number I can get to # 184XX5004 is # 1844405023, a C15A! However this could not have been a British order, and must therefore have been a Canadian contract, but with a different serial number system even though assembled possibly the same day or a day later! I can add that by May 1941 # 12 Cab CGTs had arrived in England for assembly, and were in fact assembled at the same time as outstanding # 11 Cab C15s for instance. I just wish I had a definite date!
I have been trying to work out when the # 13 Cabs came in. I have just established that I do not have a definitive answer, and if someone has the information from the parts book please shout! The earliest I can come up with is # 084210006 which is the Pilot C8A HUP, dating to February 1942 albeit with a September 1941 engine. The earliest known 1942 Model was # 284400076, a CGT. However in October 1942 at # 2844513801. the 30-degree opening windshield was changed to a 90-degree one. Gives you some idea of production at the time.