Deve you are well on your way to understanding the purpose of the octane selector. The last part of it is that when Chevrolet offered that feature, it was to allow the operator to either advance or retard the timing to account for the wide swing in octane from gas station to gas station, without losing their original pointer-on-the-BB setting.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission