Richard,
See the section "Bridge Classification Signs" at
http://www.canadiansoldiers.com/vehi...lemarkings.htm
This webpage lists a diameter of 6" for the sign itself, an confirms class 4 for Universals and class 5 for Windors. That's what the regulations said anyway, I'm sure in practice things were mixed up every now and then. As the lowest bridge class was 5 tons, it does not matter if a small vehicle displayed a "3" or a "5", they would be allowed to cross a class 5 bridge anyway. I think that's where some of the confusion comes from.
HTH
Hanno