It sounds like something that is on my 1929 AJS. The magneto gear can be adjusted without touching the taper on the end of the magneto. You loosen the nut, move the gear and tighten the nut. The magneto gear can't move when you tighten the nut as there is a pin locating it to the part on the magneto taper. The holes in the gear for the pin are not evenly spaced so you have lots of adjustment. It also allows you to check the timing without having to tighten the nut.
As this costs more than a regular gear, it's probably why later bikes don't have this feature.
