fit assembly of the two components 14 16 and subsequently control manipulation of the rotary head 16 between the two positions of fig1 a and 1b the snap fitting of the components is achieved by means of the inclusion of a radially projection engagement element 82 extending inwardly of the wall of the bore 54 and which is adapted to be disposed within the top post groove 40 when the head 16 is urged downwardly upon the post section 34 the inner surface of the radial element 82 may be chamfered as at 84 to facilitate the passage of the element 82 past the top portion 38 of the post 34 in any case it will be understood that the inherent limited resilience of the material of at least the rotary head 16 permits the above reaction between the components as they are assembled the lower portion of the head bore 54 is constructed to accommodate the head position control means 42 located at the bottom of the top post section 35 and includes a constant radius arcuate surface 86 encompassing an arc of at least 270 degrees and which is bounded by a flat cam follower surface 88 as shown in fig5 the horizontal extend of this cam follower surface 88 is substantially identical to the horizontal extent.