VI. The narrow wheels 22 shown in FIGS. 1 and 6 may be replaced by respective plastic balls made of such materials as thermosetting or thermoplastic polyurethane, polycarbonate, nylon or other suitable durable resin retained within special sockets formed of or attached to the side walls of the housings 16 or 66 or a special support for a row of such balls which is attached to the side wall portion