I've changed hundreds of these over the years but this is first time I've seen one with it's backplate. Did you older guys used to throw them away back in the day ? :
Wylex WS102 Main Switch Isolator 100A DP 2 Module
Wylex 100 AMP DOUBLE POLE MAIN SWITCH DISCONNECTOR CAT No 810 AC21
List of BS numbers for old isolators and protection Devices
List of BS numbers for old isolators and protection Devices