projection 18 as also shown it will be recalled that when the members 14 have all been advanced the frame 10 is shifted up fig3 shows the positional relationships when the frame 10 has been shifted up as can be appreciated from fig2 and 3 as the frame 10 is displaced the front face of the stop 20 engages on the rear face of the front projection 18 so that the device 16 is extended with the frame 10 the cover 17 is automatically pivoted by the extending device 16 although it is possible to open the cover 17 separately as the device 16 extends the front face 19 of the third projection 18 from the front engages the rear face 26 of the stop 25 and the stop 25 is urged inwards by the projection 18 against the force of the spring 27 to permit the extension of the device 16 when the device 10 has been advanced with the frame 10 in this manner the first and third projections 18 have their rear faces in abutting relationship with the front faces of the stops 20 25 thereby preventing inward movement of the device 16 under reactive force from the working face by swivelling the stops 20 25 through 180 u00b0 with a suitable tool the device 16 can be unlocked so that it will be retracted by the reactive pressure of the face or otherwise.