Here is a very rare photograph showing the red diamond and bar sleeve rank insignia worn on a battledress. It must be right at the time they were being phased out in the Autumn of 1941 just before the yellow upper sleeve insignia was issued.
This District Warden Incident Officer (IO) is also wearing the light blue helmet cover and appears to have the London script style IO badge above the rank badges. I'm guessing the Messenger has one of the London Borough emblems on his helmet (possibly Finchley). A unique set of insignia.
News about interesting insignia, ARP related info and period photos that turn up.