How the engine years are worked out:
It is to the best of my knowledge but could easily have mistakes. Thats why I want people to try it and see if I am accurate with bikes that they actually own.
For most models, the frame prefix and engine prefix are unique. For example the last GS150 model started with a frame number of VS5T*52364* and a motor number of VS5M*52364* for the year of 1959. It is rare that the frame and motor numbers are the same but I am assuming they reflect the same year range (ex: frame numbers VS5T*52364* to VS5T*70128* denote a 1959 bike so I assumed that engine numbers VS5M*52364* to VS5M*70128* denote the same year range for motors).
Some models dont fall into this assumption because two models shared the same motor. The TS125 and the P125X share the same VNL3M*xxxxxx* motor prefix and the Rally 200 and all further 200 models all share the same VSE1M*xxxxxx* motor prefix so heres what I assumed for this search engine:
VNL3M: There were 28744 125TSs made starting with VNL3M*001101* so I assumed the P125X numbers start at VNL3M*29846*
VSE1M: There were 41274 Rally 200s made starting with VSE1M*001001* so I assumed the P200E engine numbers start at VSE1M*42275*. There was a large jump in numbers between the P200 and PX200 so I returned the motor numbers to matching the frame numbers from the PX200 on.
I dont know for sure if any other models share engines but I suspect that the Vespa 90SS and the Vespa 90R share the same V9SS1M motor serial prefix.