There is a key to lock it closed, and the same key will lock it either on or off. In keeping with a Victorian Bell/Butler Board but not near it in the house.
I guess it must be some kind of isolator switch, but I know nothing of its history.
Looks like a antique double pole single throw switch (DPST) meaning it has two single switches connected mechanically but separate electrically (it can switch on and off two circuits at the same time). The contacts look fairly beefy so I would guess it could handle fair amount of current and the lock may have been to just keep people from getting shocked. Might have been the cutoff for the power to the bell board.
Edit: Looked around and can’t find one like that. Is there a plate on it anywhere? Could you show the cover and back of the box.