of the pin 50 and the grooves if properly positioned will accommodate the portion of the plate adjacent to the aperture 66 and in particular the semi cylindrical second part 70 of the aperture when the portion of the plate 64 adjacent to the aperture 66 is disposed in one of the grooves 58 the pin 50 is rotatable in the channel 48 but not translatable therein the depending plate 64 of the actuator 60 is provided with a slot 84 which extends along the central major axis of the plate 64 and the slot slidably accommodates a post 86 that is mounted in a bore 88 in the block 10 the bore 88 extends between the first surface 12 and the second surface 24 of the block 10 and traverses the second portion 38 of the recess 34 the post 86 extends into the second portion 38 of the recess 34 and is trapped within the slot 84 of the translatable plate 64 the post 86 limits translational movement of the plate 64 in its travel into the recess 34 to the position of alignment of the aperture 66 of the plate 64 and the channel 48 thereby releasing the pin 50 for translational movement with the actuator fully depressed the pin.