While we assume they tracked exactly what serial number bike got the change I suspect it wasn't quite that organized at Norto
I worked in the modifications dept in a UK car manufacturer a few years after Norton went bust, all changes to parts used on the assembly track were sent to the spares organisation. So as Andover Norton were the Norton Spares back in the day as well as all the drawings they should have all the changes, whether they have been kept is another question.
Sadly we don't have all of them, some of the changes on the drawings relate to the service release register but not all. AN obtained the drawings in 1991 along with tooling, sadly the change registers themselves we have no idea where they are or could be, or even how many there are as I suspect Bracebridge St had some, Plumstead had some and Marston Road had some and they no doubt got disposed of when those companies folded. Likewise we don't have all the model build books which is a shame, we only the MK3 build book resides with us.