Hello Lynn
It is the Spares box that mounts on the WW2 UC 10 or 6 line Switchboard often referred to as the battery box. It would normally hold the switchboard cords, bell, weights etc.
Geoff
A link to UC10 box you refer to is on the left in the first picture.
http://www.sentimentaljourney.co.uk/...51yzn53u05hc39