There should be a nameplate attached to the "magslips" (I'm used to calling them "synchros"), but since you asked the question I'll have to assume these aren't labelled.
I would think that 40 volts would be too low; I wouldn't be at all surprised if they require 120 volts @ 400 Hz. for most accurate following and least lag. If in fact they do require 400 Hz, I wouldn't use a lower frequency for very long at all regardless of the voltage.
It'll probably have to be somebody from the EU area to speak specifically to the "Admiralty Pattern 6550". If that's a British military specification, it should identify dimensions, torque values and a whole bunch of other information, INcluding voltage/frequency requirements.