Before you do anything double check the tax disk as they do not give year of manufacture, does it actualy say 'historic' and £nil if so then hes claiming it as a pre 73. The fact that it has a different plate could be because up until the early eighties (i think ) an imported vehicle would have been given a number plate corresponding with the year of registration, today if you choose to give up one of these plates and providing you have proof of the year of manufacture DVLA will then issue you with an age related plate, the owner may have discovered he had a very early high light import and reregistered it with an age related plate and claimed historic status. If the tax disc does not say historic then he could have bought a plate with some significance ie his initials or something. As you both own bays you have an excuse to get in conversation with him as to what year it is and how come its a later model etc , you will soon find out if his genuine or bluffing and if his bluffing put him in a headlock, bash him against the side of his van and tell him its just not cricket.