the platform 34 to receive bolts 39 fig1 or other fasteners which are extended upwardly through the slots and threaded into the bottom of the added seat 22 in order to secure it rigidly in place on top of platform 34 the elongated configuration of each slot 38 provides for adjustment of the position of the seat 22 on platform 39 the platform 34 is connected with the base plate 28 by a scissors arm arrangement which includes a pair of parallel pilot arms 40 and a rigid panel 42 the lower ends of arms 40 are pivotally connected with the flanges 30 by horizontal pivot couplings 44 which are aligned with one another to form a common horizontal pivot axis about which arms 40 can pivot relative to the base of the pedestal the upper ends of arms 40 are provided with inwardly projecting guide pins 46 which extend through horizontal slots 48 formed in flanges 36 slots 48 are aligned with one another and each slot extends less than half the length of its flange 36 the guide pins 46 fit closely in slots 48 and are able to move in the slots between their opposite ends the fit of the guide pins in the slots connects the upper end of each arm 40 with the platform 34 in a manner permitting the upper end of the arm to move linearly relative to the platform along the path defined by the slot arms 40 are.