Abstract: Stem (12) is secured to fork (6). Head (27), bearing surface (28), recessed portions (29 and 21), and bearing surface (31) successively engage the interior of housing (13), comprising first bearing (14), laterally movable locking ring (18) and second bearing (15). Locking ring (18) is repelled to the left in diametral direction against spring means urging ring (18). When shoulder (20) has arrived at the height of rim portion (32) of ring (18) the spring means are released. Ring (18) displaces to the right and portion (32) engages groove (29) thus locking the stem in the axial direction. Due to the side location of the button 9, the device can easily be handled using the palm of a hand.