A stylet unit is provided for implanting in a body, with the aid of an earlier installed tubular delivery sheath having a lumen with an inner diameter, a lead system including a distal electrode for an implantable cardiac stimulation device being slidably received in the lumen of the introducer device. An elongated main member is engageable with a thrusting region of the lead for advancing the distal electrode to a desired body stimulation site and an integral resilient knob at a proximal end serves to manipulate the main member. The knob has a nominal outer dimension greater than the inner diameter of the delivery sheath lumen and is compressible in directions transverse of the longitudinal axis. The delivery sheath can be slidably advanced across the knob which becomes compressed, enabling its continued slidable transit across the knob and, eventually, free of the lead system and of the stylet unit.