The US Army had no gas masks when the US entered WWI, and relied mainly on British supplied SBR's for most of the first year of the war. This is a reproduction of the Corrected English Model, the first US made gas mask. It differs from the British SBR mainly by the addition of the guard for the flutter valve, and the yellow painted canister.
Size 1: Head size 7 to 7 1/4
Size 2: Head size 7 1/4 to 7 1/2
Size 3: Head size 7 1/2 to 7 3/4