I always have figured that when you had the starter button on the floor, you needed three feet or a throttle button to get a cold motor started. One foot on the clutch, one on the starter button, one hand for the choke, one for the throttle knob so the motor doesn't race away. When they went over to keyed starters, so many were used to seeing the throttle knob on the dash - and they had found all these other uses for it - so it took some years to phase out the throttle knob. But the throttle knob must have disappeared in the early 60s, so yours would be one of the last years with one I would guess...
