That is a backpan? Can't find it in the dictionary and the internet searches are not really helpful. If I had to make a wild guess, I would say it is the grating behind the main vault door.

The length of the key makes really sense for this setup. The levers of my Hobbs' Protector lock ( https://www.antique-locks.com/showth...6899#post26899 ) are very similar to these levers and its key's bitting is very similar to the bitting of the unused part. So the main lock would have also very similar levers? Perhaps 9 plus 2 of the bits for turning a curtain?