Aussie box conundrum

[Kev]

I'm trying to find out what goes in the compartment in an Aussie battalion spares box (B) It is the LH compartment in the pictures below. It is a solid block of wood thats fixed in place with 2 slots at the back that go down the thickness of the wood.The block is
5 1/2 x 6 inches. I do have the list of items contained in the box , but I cant find/think of anything on a Bren that actually fits in the compartment.Anyone any ideas ?

.....it must have been made to fit something

Better still has anyone got any contacts at Singleton that could shed some light