rod 38 locates at one end of the sleeve 54 the width of the rectangular end of the rod 38 is greater than the width of the tubular aperture 56 and the sleeve 54 see fig6 as such it is too wide for it to pass through the tubular aperture 56 the other end of the rod 38 which is attached to the rear housing is located at the other end of the sleeve and is prevented from entering the tubular aperture 56 by the rear housing 12 the rod 38 can freely slide in an axial direction arrow d within the sleeve 54 the range of axial movement being limited at one end of the range by the rear housing 12 engaging with one end of the sleeve 54 and at the other end of the range by the rectangular portion 52 engaging with the other end of the sleeve 54 as the dimensions of the cross section area of the tubular aperture 36 of the sleeve are slightly greater than the dimensions of the cross section area of the rod 38 to produce a small gap between the outer surface of the shaft of the rod 38 and the inner wall of the tubular aperture 56 limited movement of the rod 38 inside of the sleeve is allowed in the directions of arrows e.