"Dylan Smith" wrote in message
...

So which base are they in?


A US registration number may have up to five symbols in addition to the
prefix letter "N". These symbols may be all numbers, one to four numbers
and one suffix letter, or one to three numbers and two suffix letters. The
letters "I" and "O" are not be used. The first zero in a number must always
be preceded by at least one of the numbers 1 through 9. The registration
number has no numeric value.



And if they aren't base 10, why can't we call
Canadian registrations C-numbers?


Canadian registrations do not use numbers. You can call them "C-numbers" if
you like, I suppose, but doing so only makes you appear stupid.