be rotated into threaded bore 20 the tip 40 is received within the channel 36 which as noted has a slightly enlarged entry 38 sized to freely accept the tip 40 as the drive member 42 continues to be rotated so as to be advanced along bore 20 the tip 40 engages the down sloping bottom surface 37 of channel 36 thereby camming the stop member 30 towards the fully extended position reached as the drive member 42 is advanced to the bottom of the channel 36 further advance of the drive member 42 causes the tip 46 to be received within recess 28 the drive member 42 thereby firmly holding the stop member 30 in place within the holder body 14 stated another way the drive member 42 exerts a camming force causing extension movement of the stop member by engagement with the sloping bottom 37 of the channel 36 with the tip 46 the stop member 30 is thereby advanced to the left upon continued descent of the drive member 42 in engagement with the bottom surface of the channel 36 thereby pushing the stop member 30 to the left into its extended position with one end 32 fully protruding from the holder body 14 as seen in fig4 the one end 32 of stop member 30 when extended is aligned below fixed stop f on the surrounding vault structure s to be restrained thereby preventing complete escape or removal of the cover c the channel 36 has upper sides 39 slightly converging.