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.
Manufacturer and model stamp on cheek "Pirelli 1931 II"
Stamp on head harness
Bag front view
Bag back view