An electrical wiring device, which may be an incandescent lamp socket, receptacle, switch, pilot light or any other wiring device intended for mounting in an opening of a metal sheet, has a mounting means of the snap-in spring type, including divergent spring arms formed with sheet engaging elements on their free ends, that have serrations adapted to engage opposed corners of the mounting opening formed in the metal sheet or plate in which the wiring device is to be supported. The serrations differ from conventional types used in springs of this type in that the portion of the spring arm in which the serrations are formed has a centrally depressed area which offsets inwardly, from the general plane of the teeth, all but the side edge portions of the portion of the spring arm in which the serrations are formed. The serrations are limited to the side edge portions rather than extending fully across the width of the spring arm as has been conventional practice, thus to promote retention of an accurate, sharp, serration. Use of a high tensile strength steel, that does not require tempering and hardening as a final production step, is made feasible by the invention.