Strictly speaking a bike should be registered by the frame number, although we see numerous bikes both in Canada and the USA which were registered before the mid 1980’s by the engine number. After that time the government obtained records from the vehicle manufacturers in their computer system of all vehicles imported. Before then they registered them by whatever number the dealer gave them at the time of sale. That is why so many vehicles are registered by the wrong year because they just registered them by the year they were actually sold.
This causes a lot of confusion because customers get wrong parts or parts which don’t fit because they are using the wrong parts books. It is essential to use the engine and frame numbers to make sure you get the correct parts books.
Also, vehicle model years run from August to July, not January to December. We frequently see, for example 1970 models, registered as 1969 models, because they were sold in the fall of 1969.
Mike's British Motorcycle Thoughts 