Hello I have never nerd anyone speak of this I'm going to guess it will keep your mag light at a constant brightness is this a Model T item Thanks Colin
These were usually and inductor (coil).
They would increase in impedance (resistance) as the RPM (and thus magneto frequency) increased.
This would act as a regulator for the mag bulbs.
As the motor increased in RPM the "Magneto Regulator" would increase in resistance keeping the mag bulbs from burning out.
Colin, you are into a treasure-trove of the most interesting items !! Thank you for sharing.
It would appear to be some kind of ferroresonant transformer.
Unlike the simple series choke "regulator", the transformer allows the use of ordinary 6V bulbs, and in the more desirable parallel connection.
Well Dad saved every thing and a lot of this stuff I don't know when he got it is just there, Cheers Colin Thanks for the information