I'm confused. A 1958 car should have a NNN XXX format or even a XXX NNN format where NNN is up to 3 numbers and XXX is up to 3 letters. If it has a suffix plate it is already looking too young (suffix A is 1962) so I'd say you have a case to get a proper age related plate. I had a Morris Minor 1960 and I have that plate on my Bike at the moment, they issued me a new one in NNN XXX format when I transferred it. So either your car is not 1958 or they messed up when they issue the new number, call DVLA and explain.