Discussion about gas detector panels on the aviation forum here, Danny:-
Gas Detector Panels
which links to a contemporary picture of armourers reloading a Spitfire with the yellow diamond behind the inboard port Browning.
http://www.flightglobal.com/airspace...17754s-jpg.jpg
To be honest I only picked up on this when my Airfix Calender wallpaper for November 2014 featured a Hurricane with a square GD panel near the port wingtip. Latest one here BTW:-
Downloads - Calendar Wallpapers Airfix