I come to you as SF's are not my norm.
I have here a smallframe with engine VMA1M012607 which makes it circa 1966.
This bike is absolutely coming out of 4th gear all the way down the street.
I believe a later gear mechanism for PK type was installed; measured 50.2 and is curved.
Correct me if I am wrong here- doesn't this gearbox take the 51mm, angular type? Piaggio #111589
