On Lucas magnetos some owners remove the peg from the back of the points thinking they have found a simple way of adjusting the timing, not thinking the peg is there for a reason!
On the cooking models here is no peg, the cam is turned to retard the timing, this in fact will maintain the armature to magnets position (max flux ) but points will open later in relation, the A/R unit alters the relation of the armature to the magnets, I wonder if set up for the cooking models is better although seems to me to result is much the same thing ?? my old brain runs out of cohesive thinking after a fairly short time these days so feel free to help me out
Now there is a good article in last months Real Classic by the young lady Bickerstaff (spelling) in which she describes an old magneto used on early American bikes where to retard the ignition the whole magneto is turned thereby maintaining the relation of the armature to the magnets and ensuring best quality spark at all times.
So I'm thinking that slotting the magneto mounts will make no difference to the results of retarding ( will still move the armature in relation to the magnets when points open) however if the slots could be used to achieve retard ( rotate the whole magneto ) then best quality spark would be achieved weather in advance or retar
