down the tread element 11 because of the leading incline 35 so that the tread element 11 can slide past as soon as this has happened the bolt is again pressed into the area of the sliding guide 12 due to the spring 34 and its lower side catches the tread element 11 at the other end of the locking element 16 the opening belt 17 is again fastened by for instance an eyelet in this variant the locking is exclusively positive and does not depend on the strength of the spring 34 fig9 shows an additional variant of the locking element in which the tread element 11 is not directly caught but rather the tensile element 8 there is a thickened part 40 on the tensile element in the form for instance of a pressed on ball as a locking element a detent pawl 41 is provided here which is seated so as to pivot on a shaft 42 and is pressed via a spring 34 into the area of the sliding guide 12 the detent pawl 41 is oriented at an incline downwards so that it likewise forms a leading incline 35 on which the thickened part 40 can slide by and at the same time pivot the detent pawl 41 against the force of the spring 43 as soon as the thickened part 40 has moved past the detent pawl the latter again pivots back and the thickened part 40 is supported on the free end of the detent pawl 41 in order to open the binding the detent pawl is in turn pivoted by an.