
Its not actually illegal to have door numbers on a car, unless that car is to be used in motorsport. I believe the MSA handbook says that their cars must have a cross through the door number when not in competition, because it is, of course, illegal to race on open public roads! But its not illegal to have numbers and letters on the side of your car, or anywhere on your car, otherwise companies wouldn't be able to print signs on the side of their vans!