Belgian Mod. E.2-339
The Belgian Engelbert Model E.2-339 was made of thin black rubber, with a speech diaphragm in the center of the mask. It had a short corrugated hose and large filter, which would be supported by a burlap sling. Most likely intended for civilian defense, this example was made by Engelbert in 1939.
Similar masks in my collection: Belgian EV.639