Canada received blocks of War Department (WD) census numbers for use on vehicles owned by Canada. The numbers featured a prefix which identified the classification i.e., Z- trucks and lorries, C- motorcycles, T- armoured vehicles including tanks and carriers, S- self-propelled artillery, etc... the prefix M was set aside for 'cars'.
Canada chose, by policy, to add the letter "C" as a prefix to the pWD Census number prefix resulting in numbers starting as CZ, CC, CT, CS and, for 'cars', CM.
As the "jeep" is actually a 'Car, 5-cwt, 4x4' and the staff car driven by the Auxiliary Services Superviser is also a 'car' in the more general sense, and that the WD set aside a block of WD numbers in the 420000 range for use by Canada, it is not surprising that it shows a CM42xxx number
Here are a few more cars. These are parked at CMHQ in London.