| It is indeed illegal to have a white square or circle with numbers on whilst driving on the public roads if not participating in an official rally and do not have the appropriate racing license and insurance. As a lot of these events have to use the public roads to get to and from stages the numbers have to be removed or crossed out when not racing.
I've actually seen one car with the tiniest black strip through the number you have ever seen. Like about a mm. Apparently this is perfectly legal!
It was acceptable in the 80's | 
