Spaced longitudinally inwardly from the tip portion 70 is another spacer-marker 68A. As shown in FIG. 8, a front section of the wire coil sheath 32 extends longitudinally to a termination point 72 where it surrounds a third annular spacer 68B and abuts against a shorter section 74 of a plastic material such as polyethylene or polyurethane and having the same outer diameter but completely surroundi