with the lock plate 24 is a bayonet lock head 63 on the extremity of the threaded rod 60 as can be seen in fig4 the lock head 63 is a flattened end portion of the threaded rod 60 obtained in a swaging operation for example the width of the lock head 63 is larger than the diameter of the bore 241 of the lock plate 24 but small enough to pass through the lock plate 4 when aligned with its axial grooves 242 thus it is possible to insert the bayonet lock head 63 of the threaded rod 60 through the bayonet lock plate 24 and to create a locking action by rotating the threaded rod 60 approximately one quarter turn as can be seen in fig2 and 4 the bayonet lock plate 24 is arranged at a certain depth inside the core body and supported by the material of the rigid inner core body part 20 for example the bore 23 which leads to the bayonet lock plate 24 has a first bore portion 231 above the bayonet plate 24 and a second bore portion 232 underneath it the upper bore portion 231 has a cross section which corresponds substantially to that of the lock plate 24 thus serving as a guide bore for the lock head 63 of the threaded rod 60 unlike the first bore portion 231 which is formed by the rigid body part 20 the lower bore portion 232 is formed by a portion of the softer core body jacket 40 this bore portion therefore need not have the.