In this schematic the "Clip B12,tube" is used most often.
Check out any of the nineteen Radiator Shroud - Reserve Tank components enlisted.
Parts such as the Radiator Shroud - Reserve Tank: Rubber,fr-fend.mt, Clip B12,tube, Pa,radiat Shroud, Reflex Refl Fr are included in the Cx500 1981 (b) Italy Radiator Shroud - Reserve Tank parts fiche.