The butler is innocent until proven guilty.
The challenge with part 3 is that the files dealing with the development and DND acceptance of what became the cab 11 are not readily found. With the declaration of war there was an obvious need for additional MT, however, the Brits had indicated that they didn't need anything from Canada and that they were best suited to equip the Empire. Let's not forget that Britain didn't become a 'nation of merchants' by buying goods overseas. This attitude only changed post-Dunquerque.
The early-war files that I do have clearly indicate that there was still a preference, from both manufacturers, to produce modified commercial pattern (MCP) vehicles to meet the relatively small Canadian demand of a few thousand vehicles.
By end of 1939 the two manufacturers were still paying lip-service to the concept of integration of design and standardization of parts. When (read - 'if') the relevant files are uncovered we may find that the cab 11 design was based more on a combination of the slavish acceptance of the 13 inch rule imposed by the British War Office and whatever parts/tools were available to Ford and GM.
I shall continue researching and, if not the butler, then perhaps the guilty party is Col Bloggins, in the study, with a candelabra.
Clive