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.
News about interesting insignia, ARP related info and period photos that turn up.