would constitute an electrical connector which would be connected to appropriate terminals of electronic circuitry although the detector has been illustrated in fig2 and 3 as a mechanical switch it will be apparent to one skilled in the art that a light sensitive switch or a magnetic switch could equally well be used it is only necessary that the detector have two states and that the movement of the locking rod be adaptable to change the detector from its first state to its second state or vice versa referring now to fig5 the cylindrical drive unit assembly comprises as in prior art such assemblies a retractor assembly housing 101 the retractor assembly housing 101 is modified in accordance with the invention to include an opening 103 and an outer flange 105 the purposes of which will be discussed below the retractor assembly housing 101 also includes a front wall 106 the cylindrical drive unit assembly also includes a retractor assembly retainer cap assembly 107 having a cylindrical extension 109 and a plate 111 when the cylindrical drive unit assembly is assembled the plate 11 abuts the inner surface of the front wall 106 a switch 113 is mounted on the front surface of the plate 111 as described below the switch 113 includes a plunger 115 and openings 117 and 119 when assembled the openings 117 and 119 are.