Determining voltage requirement of unknown circuit board
Look at your old circuit and you will probably see lots of electrolytic caps .Note the voltage ratings .Try to find a cap across the supply rail .Now replace the caps.Slowly run the unit up on a lab supply with the volts starting  at zero. Carefully raise the volts while observing the ammeter using the cap voltages as a sensible guide for Vmax. The unit was running of dry cells 1960s so it wont consume lots of current .As a guess it will start running at about 6V and the nominal volts is 9V