metal housing provided with a sidewall and a neck connected to the sidewall via a shoulder , the neck being constructed and arranged for irreversible fixation to a dispensing head for perfect leaktightness to liquids . the dispensing head includes a plastic flange provided with a sealing skirt and an outer catching skirt provided with a snap - fit rim on its inner surface , wherein the neck has a wall and the shoulder has a wall , both said walls being of substantially the same thickness as the sidewall .