a spool assembly 5 the spool assembly is characterized by a hollow and generally cylindrical portion 13 having a first end 14 and a second end 15 and being rotatably mounted within the housing the generally cylindrical portion has an outer region which provides an external tether supporting surface 16 and an inner region which is delineated by an inner conical zone 17 formed with spaced ribs or splines 18 along the circumference thereof the diameter of said zone decreases in an axial direction from the first end to the second end the spool assembly is further characterized by first and second opposed guidewalls which extending perpendicularly to the axis of the spool assembly and are positioned at each of said first and second ends respectively the first guidewall 19 is adapted to receive a spool locking means 7 and said second guidewall 20 is adapted to engage spring biasing means 6 the guidewalls are further characterized by bearing surfaces 21 and 22 extending axially therefrom and are adapted to facilitate ease of rotation of said assembly within said housing in the particularly preferred embodiment as shown thrust bearings 26 and 27 are included which aid in maintaining the positioning of the spool assembly during the operation thereof the spool locking means has a cylindrical member 23 adapted to frictionally engage the inner conical zone of said spool assembly and additionally has corresponding ribs or splines 24 spaced along the outer circumference of said member upon depressing the actuator knob 12 which causes insertion of the spool locking means into the housing and into frictional engagement with.