If you can't find an answer then I'd try sticking a 3v battery in (you can easily make an insert from a small block of wood or plastic with a bit of copper tape or wire as a conductor). If you don't see any life then it's probably intended for a 6v battery.

Using too low a voltage shouldn't harm anything, designers have to plan for a dying battery delivering too low a voltage. Usually you get dim LEDs, a meter that reads oddly, or similar.