Let's make it short. Berliet, Panhard, Chausson, Isobloc, Floirat and Rochet-Schneider are wrong answers. We have to find a manufacturer of buses and coaches from the late 1930's to the mid 1960's, located west of Paris.
Bernard : too old; the first buses date from the 1930's, built in Arcueil, south of Paris.
Brossel : a belgian bus, first buses dating from the 1930's.
CBM : located in Le Mans.
Chenard & Walcker : seems to be too old, buses dating from the 1930's.
Delahaye : located in Tours, first buses dating from the mid 1930's to the late 1940's.
Gruau : first buses dating from yhe beginning of the 1950's.
Heuliez : first bus dating from 1949.
Latil : bus production between 1910 and 1939.
Saviem : bus production between 1957 and 1977.
Somua : bus production from the beginning of the 1930's to the mid 1960's. A good candidate, then. But the buses were built in Saint-Ouen, north of Paris
Verney : bus production between the beginning of the 1940's and the mid-1960's. I don't know where they were built.
I suppose that Citroën and Renault are excluded, being too well-known. Therefore, I'm stuck.