Mid 60s, was 67 or 68 that 6 speeds became the maximum allowable along with a maximum number of cylinders depending on engine size on the grounds of safety even though the restrictions were more about money, all the Japanese factories pulled out of running works bikes when that happened
From memory 1 of the little Hondas pulled 20 000+ rpm & had 18 gears, 6 speeds inside the box & a 3 speed box externally similar to what pushbikes had controlled by the left twist grip.
Truly a time of pure racebike engineering