[broadgage]

AFAIK these types of clocks were rated by CURRENT and not by voltage.
An original installation would contain numerous clocks and long cable runs, maintaining the correct voltage at each clock would be a challenge, but a constant current fairly easy.
Clocks of different sizes intended for use on the same system would all have the same operating current, but probably a different voltage drop.

For a new installation, the total resistance could be measured or calculated and the battery specified to drive the required current through this resistance.
A lead acid battery on continual trickle charge was the traditional power source.

The original master clock was generally a good mechanical clock, corrected regularly by radio time signal or astronomical observations.

I agree that this clock would be better restored to original working condition, useing a modern electronic unit to provide the pulse signals.