extends upwardly along a rectangular insert opening around the interior periphery of the frame body opening 52 39 defining the belt receiving slot 52 the liner 100 has a thickened rear portion 106 with a channel 108 for receiving a rear bar portion 56 of the frame body 101 behind the slot opening 52 39 and has a pair of hollow cylindrical projections 113 in front of the lip 104 that extend through a pair of holes 114 in the body flanking the channel 53a the lip 104 has a gap 104a in the region of the channel 53a leaving the channel open to the rest of the frame slot 52 the liner 100 is applied to the frame body 101 by slightly deforming the liner and sliding it rearwardly along the under side 105 of the frame body 101 so that the channel 108 engages the bar portion 56 and then snapping the front of the liner upward inserting the projections 113 into the holes 114 the front surface of the rear portion 106 of the liner 100 provides one of the walls 58 along which the belt 15 slides the snubber slide 60 is adapted to be fitted over the frame 50 prior to the belt being inserted through the slot 52 the snubber slide 60 as best seen in fig4 and 5 is a closed channel shaped body.