respective mast stage and each bottom pulley 72 extends through a cutout 75 in the back wall 41 of the respective mast stage the rear stationary mast stage 35 has a single upper pulley 76 journal mounted on an angle bracket 77 mounted on its front wall and extending through registering cutouts 78 in the front and back walls thereof the carriage 31 has a pulley 80 extending through a cutout 78 39 in its back wall the top pulleys 71 are journaled on shaft bolts 71a mounted in blocks 82 fixed by bolts at 83 to the back wall 41 of the respective mast stage and the lower pulleys 72 are journaled on shaft bolts 72a passing through blocks 84 fixed by bolts 845 to the front wall 40 of the respective mast stage the blocks 82 84 have front beveled faces 82a 84a respectively angled so that the respective pulleys are tilted in an upright plane the two upper pulleys 71 are tilted such as to extend rearward into the right portion of the center passage 85 along the column portion 38 of the respective mast stage from the left passage 55 in front of the stage the two lower pulleys 72 and the carriage pulley 80 on the other hand angle rearward from the right portion of the central passage 82 to the left passage 55 which is next to the rear this positions the pulleys such that the cable 72 extends from a stop 86 on the upper end of the front mast stage 37 travels under the carriage pulley 80 then over the top pulley 71 and under the bottom pulley 72 of the extensible mast stages 37 36 progressing from front to back then travels over.