Thanks for the info
The open question is still the year of manufacturing. I found at
https://www.brundrit.co.uk/british-military-keys a similar key of 1928 that has a serial number in the 368. But the year JC is not clear to me
Indeed that disk should be to protect from HV. If it was used to key (AM Modulate....) a spark transmitter, quite high and lethal voltages and currents were switched. I wonder if the sparks got old in those days.....