The magnetic selection ring has a ferromagnetic body defining a flange provided with recesses. Slots filled with diamagnetic material are provided in the recesses to form heads each accommodating the core of a respective release electromagnet. The selection ring is usable with selectors of the type provided with a shaped elastic portion, a lower heel, which is outwardly flexible for being affected by a lifting cam or inwardly retainable to avoid interference with the cam, and an upper heel, actuated by a lowering cam formed on the lower surface of a complementary ring for retaining the elastic portions of the selectors. The selectors have a lowermost tapering tab, which can be moved by a retraction cam into contact with the magnetic ring. When the tab is retained by the permanent magnets, the heel assumes the configuration for not interfering with the cam. The electromagnets are activatable for selectively releasing the selectors.