I don't know the coachbuilder, but here is some information that might be useful for other puzzlers. According to the text written on the body, this car was most certainly used as an advertisement for a second-hand car dealer located at 188 avenue d'Italie in Paris (nowadays there's a contemporary building in this location, with an Opel dealer on the ground floor). "KEL 30-30" was the phone number of that car dealer : the "KEL" prefix was used only between 1957 and 1963.