Landline Socket with ASDL Filter (Brunswick BN3)

This is a cover which fits onto the box on the wall.
The filter means that a network cable and a normal phone cable can both be plugged into it.
As far as I know it all works perfectly well, the BT engineer left it behind when he upgraded the socket in my flat with a new cover. Don't want to just chuck it if it can be of some use to someone.