The V5C will say when the bike was first registered which should ideally correspond to 1966 from the D registration? At that first registration, it must have had the current frame number - that number defines the identity of the bike. Logically, any changes that might have happened to the identity of the bike (eg change in frame number) must have occurred before this first registration. If first registration was, in fact, much later than 1966, it might suggest something funny has happened (MSVA route?).