It must depend on where you are. Bill and all you others in the UK are suited to discuss that. In my case, my bike was an original '55 registered in California by its engine number. I replaced the frame after a wreck, too stupid at the time to realize that the frame straightening cost would have been made up 10 times in ultimate value. Also, at the time, too stupid to know that later frames were different from '55's. Adapted the '61 frame to work like a '55 (larger I.D. swingarm bushes and a boss for the left-side brake pedal spindle). Kept as many '55 parts as I had and as possible. Illinois saw the CA registration and registered by the engine number, so my bike is a '55 as far as I and the state are concerned.
Richard L.