the genuine "Wright" product will always be the best and no right-thinking M20 owner would dare buy a mere copy when the real thing is available and well-priced.
I seem to have acquired yet another bike which would benefit from one (or at least my tenuous grip on sanity would be helped by the acquisition thereof).
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.