The Belgian A.G.5 was a very similar to the Italian P.43, with twin cheek exhale valves. This example was made by L'Anti-gaz in 1936.
Similar masks in my collection: Italian P43
Manufacturer stamp on cheek 'ANTIGAZ TYPE AGREE ARLE GOUVERNEMENT N103 II"
Filter top view
Filter stamps bottom view "W. 35 P BREVETE" "P.V. No 102" "TYPE AGREE PAR LE GOUVERNEMENT"
Inspection and date stamp "L.P.A. CONTROLE S.P.G. 1936"
Manufacturer logo "L'ANTIGAZ S.A. BELGE"
Metal canister top view "ANTI-GAZ"