[Alex van de Wetering]

But if the census number was not preceded by a “C” it was not a vehicle produced under a Canadian contract.

I don't think I agree; that might have been the "rule", but period pictures show that the reality was often different. There are plenty of pictures of Canadian vehicles without the "C" prefix, both in the 42..-43 range, as well as higher numbers starting with 55, 56 or 59. The 42..-43 range as we all know was for Canadian contracts, but even with the picture of the CBC HUP above, you can see that the "C" was often omitted, and therefor in my opinion, you can't say a vehicle is British contract by just the lack of a "C" prefix.
Especially in late war pictures you see a lot of Softskins without the C-prefix, but with Armour the lack of a "C" seems more common, like T16 carriers, Shermans, Staghounds.

And:
• SM = Supply Mechanical, these were British contracts.
• CD and/or LV = Contract Demand (Land Vehicles (?)), these were Canadian contracts.
Read more here: http://www.mapleleafup.net/forums/showthread.php?t=1965

Thanks Hanno!