Derek,
I won't go into too much detail but in the 1980's there was a time limit for all registrations to be notified to DVLA in order to have them put onto the "New" database. There were vehicles in garages and museums that had an old logbook and their registration numbers became obsolete if the owners missed the deadline.
When the vehicles that missed the deadline were eventually put on the road they were given an "Age Related" number even though the owner sometimes knew what the original number was. Some years after that, if you could prove the original registration number, the government relented and allowed the original number to be reallocated to the machine if the new owner wanted to go down this route and pay the appropriate fee for the privilege. In these cases, the actual date of re-registration was put on the registration document (7.4.98 in your case) and there was normally a note somewhere on the document that stated something like "Declared year of manufacture 1958" or whatever year the vehicle was made.
There are other reasons for the sort of anomaly that you have but the example I've given will be the most likely.
Hope this helps,
Beezageezauk.
