Bennett - I've always wondered why too. Perhaps it was cheaper to manufacture with a stop motion screw. There is an extra cast iron component with the notch system, but not that that would make a huge difference I would have thought.
Now for the bobbin winder. There is no moving arm to take the thread from side to side. Instead it passes over the curved edge of the metal plate shown in the second picture. Somehow (the dynamics of it all completely passes me by) it winds from side to side, and changes direction as soon as it reaches the notch at the side. It winds a beautifully tight and even bobbin - much better than any Singer I have.
Must try and remember to dust the machine before taking pictures rather than after.