position with respect to base 6 and has the ability when desired to permit relative rotational motion therebetween this detention means comprises a knob 82 which has a protruding operating lever 84 if desired the relative dimensions of the parts can be such that they fit very snugly and the knob 82 can be positioned by the lever 84 and retained in any position between the limits of the opening 92 in the seat body 4 because of the snug fit in this manner the depending teeth 112 of the knob 86 can be positioned to either be vertically extending as shown in fig6 or at an angle to the vertical as shown in fig4 the knob is provided with stub shafts 102 and 104 which are snap fit into assembled position into the recesses such as 97 in the side depending walls 94 96 of the intermediate member 4 alternatively torsion springs may be provided for the knob 82 so as to provide bias to urge it into a desired 34 normal 34 position these springs 90 91 are positioned one at each end of the knob 82 as more clearly shown in fig6 and 7 the torsion springs have legs 93 and 95 which are retained between a post 98 or 100 on the side walls 94 96 respectively and a pin on the knob such as pin 86 shown in fig6 a similar pin would be provided for the opposite side of the knob 82 fig6 shows the.