Hmm, must have missed this one before.
Anyway, I`m no expert on early flashlights or the batteries they run on (is that battery pack an original then? normally those old carbon cells corroded away didn`t they?) but I reckon the simplest way to find out your light`s voltage is by trial and error. Hook a single AA cell to it- the zinc carbon type not the new fancy lithium ones cos their voltage is ever so slightly higher- improvise connections if you have to. Then see how bright the bulb is. If it`s dim, you`re probably safe to try 2 AA cells in series. If it`s fairly bright, you`ll probably only need one cell (or 2 identical ones in paralell). Unhook it right away if it`s bright white to prevent any damage- but that`s doubtful on just one cell.
If you have access to a variable power supply that goes from 0 volts upwards, smoothly variable then you could try that- keep turning the voltage up until you think the bulb is at about the right brightness, then take note of the setting. I`m sure you know that vintage flashlights don`t burn as bright as modern ones do- take care. It`s all too easy to turn the volts up too far and fry your poor light`s bulb.
But I doubt any flashlights would have operated on less than 1.5 volts so you`re probably OK with the battery test.
