e means or structure for adjustably and securely setting and fixing the segments 11 of the supporting member precisely onto the predetermined portions of the upper surface loaded surface of the base member 2 fig6 to 9 show examples of such means for locating and fixing the segments of the spring retainer supporting member namely according to the invention one or as required two or more projections of suitable width height and length are formed on either one of the lower surface of the spring retainer 7a 7 39 c or 7 39 e and the upper surfaces of the supporting surfaces 11 1 11 2 11 3 see fig6 and 7 while corresponding recess or recesses of the width depth and length corresponding to those of the projection or projections are formed in the other alternatively the projection or projections are formed in either one of the lower surfaces of the segments of the spring retainer supporting member and the upper surface of the base member 2 see fig8 and 9 preferably the size of the recess is selected to be somewhat greater than that of the corresponding projection in order to facilitate the insertion and withdrawal of the projection to and from the recesses theoretically only one combination of projecton and recess suffices for each segment of the spring retainer.