a first guiding means a set of guiding apertures 70 extending through the plate 60 are adapted to act as a second guiding means the guiding apertures 70 are adapted to receive a corresponding set of guiding rods 72 extending downwardly from the inner surface of the top plate 74 of the cover 56 thus aligning the cover 56 with the mounting base 12 a pivoting supporting bar 76 is pivotally mounted to a lower segment of the plate 38 by a pin 78 extending through an hinge 80 the supporting bar 76 has a relatively pointed end 82 adapted to be releasably inserted inside a corresponding notch 84 provided in a lower wall 86 of the sleeve 16 the supporting bar 76 is adapted to pivot between an inoperative position wherein it lies flat against the vertical surface and an operative position illustrated in fig3 and 6 wherein the pointed end 82 is inserted in the notch 84 for supporting the sleeve 16 in a substantially horizontal position referring now more specifically to fig8 through 12 the sleeve 16 has a top body 88 and a lower wall 86 releasably fixed to the top body 88 by conventional fastening means such as screws 90 extending through conventional apertures 92 the top body 88 has a channel 94 extending longitudinally therethrough and adapted to receive the internal components of the supporting structure 10 the channel 94 is adapted to receive a guiding hollow jacket 96 the coaxial shafts 18 and 20 are adapted to translate inside the jacket 96 the longer and diametrically thinner shaft 20 is.