Abstract: Two reversing pulleys are spaced apart transversely to their axes. At least one of said pulleys is a drive pulley. Each of said pulleys comprises first and second axially spaced discs, each of which is formed with radially outwardly open recesses, which are angularly spaced and are defined each by side faces which are convexly curved like sides of gear teeth. A series of slats of predetermined length extends along an endless path of travel between and around said reversing pulleys. First and second sets of connective members are respectively aligned with said first and second discs along said path of travel and disposed on opposite sides of said series of slats. A plurality of pins are mounted on said slats on opposite sides thereof at each end edge portion of each slat and in adjacent ones of said connecting members.