The photo posted by Peter Balls appears to be the same as mine.
The knob screws in and out to set the throttle.
A length of lawn mower control cable would have to be crimped into it.
It acts on a lever clamped on the cross bar part of the pedal linkage.
The cable passes through a little brass sleeve with a hex head set screw to hold it.
I suppose Lucas was in the business of making dashboard knobs and this was right up their alley.
Sort of an early version of cruise control, although they didn’t call it that.
I have not driven mine yet on a long quiet road where I could try it.