My Mk3 Seeley has 1966 stamped on the steering head. The position of the motor is important. I do not know how far forward the motor can be in a Mk2 frame - also with the frame tubes in front of the motor, there is the same problem as with a featherbed frame, of the exhaust pipes missing the down tubes, when using a 2 into 1 exhaust. I am fairly certain Gus Kuhn used the Mk3 frame - my bike has the 850 motor , not a 750 - it is not strictly eligible for the pre 1973 race class. But in historic races nobody is ever silly enough to protest about somebody else winning a race.
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