I checked on my 601 and it appears that the black rectangle is a tensioning device. The screw threads into the chassis and adjusts the amount of tension that the rectangle applies to the two wheels on the knob assy. If you look up inside, there is a slot that the rectangle goes into. Hold the rectangle in position and then start the screw with the head of the screw holding the rectangle in place while tightening the screw will increase the pressure on the wheels/rollers.
Edit: forgot to add that the screw passes through the u-shaped cutout in the rectangle.
Last edited by Fred Aspen; 01-01-2013 at 01:17 PM. Click to view previous post history.