Belgian EV. 639
The Belgian Engelbert EV. 639 was made of thin black rubber, with plastic eyepieces and a portruding speech diaphragm. It used a 25mm filter attached directly to the mask. Most likely intended for civilian defense, this example was made by Engelbert in 1940.
Similar masks in my collection: Belgian E.2
Manufacturer and model stamp on cheek "Englebert E.V. 659 2"
Stamp on cheek "TYPE AGREE PAR LE GOUVERNEMENT BELGE SOUS LE P.V.147-172
Inspection stamp inside mask "22 M.D.N. CONTROLE S.P.G. 1940"
Filter stamp top view "BREVETE F-39-P"
Filter inspection stamp side view "4.M.D.N. CONTROLE S.P.G. 1940"
Filter manufacturer label "ENGLEBERT & Co LIEGE P.V. 172 TYPE AGREE PAR LE GOUVERNEMENT"