This is true, British Army vehicles did carry "tax discs" during WW2. I think it may not have been carried though to the end of the war. As pre-war military vehicles, inc RAF, carried civilian reg. numbers especially allocated, I think these discs were a form of exemption. The disc holder was a rather nice diecast item, which was screwed to the bodywork, the back of it had a screwed disc, which removed to access tax disc. They had W/l\D cast on them and were at one time still found at militaria sales.