Gas Detection Brassard
The gas brassard was another British made item that was used to the US Army.
They are made of a strong paper material which has a reactive coating applied. When vesicants were detected the brassard would turn a red/pink colour.
They are commonly found folded in the bottom of gas mask bags and through age the paper goes quite stiff.
They were issued to all forces on D-Day, as the Allied Expeditionary Force commanders were convinced the Germans would use gas as means to repel the attacking force.