I am now in a position to answer my own question!
By checking the excellent resources at Camera Eccentric I have tracked this item down to be a Marion 'Soho' Changing Box which was made to accommodate film formats 2.5x3.5 inch, quarter plate, post card, 5x4 and half plate. In (I would guess 1920s) this item would have set you back a not inconsiderable 5 10/-
I am now looking for a Marion Soho in 5x4 format to go with it. A long term view needs to be taken on this one methinks.