Belgian M1931
The Belgian M1931 featured a rubber facepiece, two 50mm glass eyepieces, and a corrugated rubber hose leading to a large metal filter canister. It was used by the Belgian military for almost two decades. This example was made by Pirelli in 1931.
Similar masks in my collection: Belgian M1934, Belgian AG15, Belgian AG20, Italian S.A.B. M33, Italian I.A.C. M31, Spanish M33