The octane selector doesn't have a knob; a bolt holds that bracket to the block. To use the octane selector, you would loosen the bolt and move the whole bracket to change the spark timing to eliminate detonation (or cause it, if you go in the wrong direction) that results from using fuel that has a lower octane rating than what the ignition timing was originally set for.
Where is the knob located? It sounds like it is holding the octane selector bracket to the block, in place of a bolt. Is that the case? Someone in the past may have replaced the original bolt with that knob in order to make it easier to adjust the timing with the ocatane selector- but the original bolt was pretty easy to loosen.
Harvester
