I am pretty sure that I know what the white knob is. It is part of something called Magnastitch. It is a magnifying glass that snaps onto that knob. You could buy extra "knobs" so that you could put one on each of your machines. They attached with adhesive. If you wanted to remove it you could drizzle GooGone behind it and it would eventually come off. Make sure you allow the GooGone to soak in. Otherwise you could accidentally remove the paint/finish on the machine.
Colleen