the upper surface of the bottom wall 32 in vertical alignment with the bracket 37 an elongated roller 41 is rotatably mounted on an axle 42 carried by the bracket 40 and such axle is positioned generally parallel to the front wall 21 and at least slightly below the lowermost end of the opening 24 the bracket 40 is positioned so that the periphery of the roller 41 is contiguous to the rear surface 28 of the front wall 21 a flexible substantially imperforate belt 43 is trained about the upper and lower rollers 38 and 41 in a manner to provide front and rear runs 44 and 45 respectively the belt is substantially wider than the opening 24 in the front wall and the front run 44 slidably engages the rear surface 28 of such front wall for a purpose which will be described later the mounting bracket 36 is positioned between the back wall or panel 17 and the mounting bracket 37 and forms part of a fixed pulley 46 a grooved sheave 47 is rotatably mounted on an axle 48 carried by the bracket 36 and such axle is substantially parallel with the axle 39 of the roller 38 as shown best in fig3 the bracket 36 supporting the grooved.