What is it and where does it go?

[Michael R.]

C31UCW 104376, assembly of parts with four wood screws, one bolt, lock washer and nut.
C31UCW 104375, bracket
C31UCW 104374, Socket, BREN gun support

FUC-03, pages 325.
When the fire extinguishers are mounted inside the gunner loop hole plate on the MK-II*, the former weapons bracket with associated butt holder on the floor for the MK-I* became redundant. The gunner right handgrip of the MK-II* has an extension fitted to injure your knee, or perhaps to hold the BREN in the transport position, with the muzzle in the wood C31UCW 104374 socket.