94 extending substantially the full width of apparatus 10 and having chain sprockets 96 96 39 secured to its opposite end portions an elongate chain member 98 fig1 is entrained about sprocket 96 and about idler sprockets 100 mounted in any suitable manner upon frame 12 beneath drive sprocket 96 and has its opposite ends respectively secured to opposite end portions of side wall 80 of carriage 76 at the other side of apparatus 10 see fig2 a second chain 98 39 similarly is entrained about drive sprocket 96 39 and idler sprockets 100 39 and has its opposite ends secured to side wall 80 39 of carriage 76 when worm 90 is rotated in a 34 forward 34 direction by drive unit 88 chains 100 100 39 moves carriage 76 forwardly from its rearmost position shown in solid lines in fig1 to its forwardmost position indicated by phantom lines upon reversal of the direction of rotation imparted to worm 90 by drive unit 88 carriage 76 moves rearwardly back to its illustrated solid line position a plurality of elongate slots 102 fig4 and 5 extend through base plate 78 of carriage 76 slots 102 are arranged in a plurality of longitudinally spaced parallel rows there preferably being eight such rows each of which spans substantially the full width of carriage base plate 78 one complete row and parts of two others are shown in fig4 the number of slots 102 in each row illustratively nine corresponds to the number of spaces between adjacent ones of the parallel grid bars 32 defining the upper extremity of passageway 14 the slots 102 of each row directly overlie respective ones.