So, as per the larger picture in the above image plate, they developed a snap-on lid that could be removed and snapped onto the back of the unit, with two rubber protrusions on the top.

Readjust the top and bottom guidelines so that they are once again snap to the top and bottom of the background image.

This is the basic loop with a roll (usually a snap roll) at the top of the loop. The roll has to be centered at the top of the loop.