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[UK] Found during house renovation
(i.imgur.com)
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.
Both switches would turn off/on at the same time. If one was a door bell and the other the butler bell, this switch would make them both ring at the same time.