described in more detail below are movably secured to outer support assembly 27 primarily by means of slots defined in the sidewalls of recesses 28 and 29 and protrusions from the supports engaged in such slots more specifically it will be noted for instance from fig2 that the linkages and traction members extend symmetrically from either side of central plate 25 and are connected directly or indirectly to central plate 25 by means of biasing springs 32 and 32 39 in conjunction with pivots 35 and 35 39 throughout this discussion elements shown in the extended right side of fig1 position will be identified with numerals having a 34 prime 34 while corresponding elements in the retracted position will be identified without benefit of the 34 prime 34 links 34 and 34 39 attached at pivots 35 and 35 39 to central plate 25 connect to members 36 36 39 respectively at slot 37 defined in members 38 and 38 39 pivots 33 and 33 39 connect members 36 and 36 39 to outer support assembly 27 members 38 and 38 39 are connected to extendors 40 and 40 39 extendors 40 and 40 39 also carry end support means 44 and 44 39 thus when extendors 40 and 40 39 are displaced by members 38 and 38 39 end support means 44 and 44 39 guide the movement of the assembly along slots 74 and 74 39 shown in fig4 until the dogs 45 and 45.