the second through hole 432 a diameter of the second through hole 432 is substantially equal to that of the first through hole 232 of the second locking leg 20 a width of each of the second slot 333 and the third slot 433 is slightly greater than a thickness of the plate 23 of the second locking leg 20 the third slot 433 extends through a middle of the teeth 431 referring to fig1 the assisting member 30 is arranged on the connecting portion 13 of the spring member 10 between the connecting plates 131 wherein the second slot 333 of the assisting member 30 corresponds to the first slot 133 of the connecting portion 13 the actuating member 40 is then supported on the inclined top surface 33 of the assisting member 30 to ensure that the teeth 431 engage with the teeth 331 the top end of the plate 23 of the second locking leg 20 passes successively through the first slot 133 of the connecting portion 13 and the second slot 333 of the assisting member 30 and enters into the third slot 433 of the actuating member 40 thereafter a pin 45 is inserted through the elongated slots 132 of the connecting plates 131 the second through hole 432 of the actuating member 40 and the first through hole 232 of the second locking leg.