the portion 38 desirably being solid and essentially rigid and non deformable between those faces to take relatively heavy clamping load forces without substantial deformation when the slide tightly grips the cords extending rightwardly from its main gripping portion 38 the slide has a reduced width neck 43 to which there is connected a handle portion 44 forming a handle loop accessible at the outside of the body a user can insert a finger through this loop and pull on the slide element rightwardly in fig1 and 2 to move the slide from the locking position of fig2 to the released position of fig3 extending outwardly from the juncture of main locking portion 38 and reduced neck 43 the slide carries and forms two similar laterally projecting feeler arms 47 which may be of relatively small cross section to give these feelers considerable longitudinal flexibility at their free ends the arms 47 have toothed portions 48 each desirably having two teeth as shown which are engageable laterally outwardly against the inner sides of the two cords 13 arms 47 are resilient and normally tend by their resilience to return to the broken line positions illustrated at 47 39 in fig2 when the slide is contained within body 11 these arms 47 of course cannot return.