An excellent portrait of an early-war Gas Identification Officer - GIO - designated by two black diamonds on a yellow helmet. One black diamond was for an assistant to the GIO. Three black diamonds were for Senior Gas Adviser. From 1942 helmets were standardized across the country:
Gas Adviser - Senior Gas Adviser
GIO - Gas Identification Officer
GI - Assistant to GIO
FOOD - Food Decontamination Officer
DC FOOD - Food Treatment Squad
News about interesting insignia, ARP related info and period photos that turn up.