The Belgian A.G.20 facepiece was identical to the A.G.15, including the embossed "A.G.15" stamp. But instead of having a small filter attached directly to the mask, there was a corrugated hose leading to a much larger filter. Most likely intended for civil defense, this example was made by L'Anti-Gaz in 1939.
L'Anti-Gaz = "Anti-Gas"
'L'ANTI-GAZ S.A AGREE PAR LE GOUVERNEMENT AUTORISATION No121"