I believe this is exactly the as fitted to Alvis Speed 20 models that used dual ignition. I has three positions, the middle for "Off" and one either side for either mag or coil. The spindle comes through the back to be connected to the H.T. change over switch on Alvises.It is marked "AM" under a crown but it is not an Air Ministry mark as far as..
An original Bosch ignition coil, marked TF6/1. It is key operated (no key) at one end and has 6 connections on the bakelite end. Complete with original mounting bracket. Buyer pays shipping costs.