These three sides are the cover of the magnetic structure and of other operating members, which is made of an appropriate material, e.g. of plastic or the like, and has a recess 401, in a predetermined area, for accommodating the fastening base of a receiving coil (see FIGS. 15 and 16). [0061] The free end edges of the upper and lower sides 101 and 201 are rounded, preferably with a semicircular p