This wonderful photograph of nurses also shows a scarce ARP chaplain's helmet. A similar armband was also issued.
A very nice condition 1939-dated WW2 ARP Chief Warden's helmet. This particular item was sold on eBay for £165 (September 2018).
An interesting and unusual helmet came up for auction; a 1941-dated bakelite "Controller" helmet with two white central strips.
The Fire Guard service developed from the Fire Watchers that were volunteers (often forced volunteers if truth be told...) who spent long nights watching over industrial and commercial properties. They would deal with incendiary devices and call out the fire brigade if necessary. By 1944 the Fire Guards were well organised and had a variety of ranks. The below table for ARP Memorandum No. 17 dated 1944 shows the various markings that then existed within the service.
This often reproduced list of London's Civil Defence helmet markings first appeared on page 1276 of Volume 4 of The Second Great War by Sir John Hammerton and Sir Charles Gwynn, The nine volume history appeared shortly after the war (1947). It shows the wide range of helmet markings but there were no doubt many, many more.
News about interesting insignia, ARP related info and period photos that turn up.