To this end anti-friction means are provided in the form of cord-like ball bearing retainers 146, see FIG. 4, which may be formed of metal but are preferably formed of a plastic material such as nylon, vinyl, polyvinyl chloride (PVC) or polypropylene, each cord-like retainer comprising a central core 147 substantially circular in cross section with integral, laterally projecting ribs 148--148 exte