by JerrryR » Fri May 24, 2019 8:40 am
Hi Simon,
Based on observation with 2 specific data points, I believe June (F) was the transition month. I have examples of F3 frames that were the last of the 53Ks and first of the 54Ks. I also have examples of F6 frames for the last of the Ks and the first XLs. I can't say this practice was a factory policy and would be open to frame model year changes happening in other than June. For example the asymmetrical seat casting with the right underside boss for mounting the 65H coil and the 65CH voltage regulator happened about April of 64, shows up on the late 64 model year frames. Likewise the VIN boss for the 70 models appears to show up about April of 69. I think that June most likely is a good bet for significant model year changes like the rake angle from 53 to 54 or the neck raised between 55 and 56. For changes that were compatible between model years I believe you could see the change occur late in the previous year as they used up remaining parts' and transitioned to the new ones.
Have a great day,
JerryR