Fascinating video! Right at 2:52 - that round plastic piece you found. I'm guessing, but I believe that's a hand sewing needle holder. It once might have had a circular paper cover with needle sizes listed, corresponding to the notches. You'll probably find a hole on the edge somewhere so you could "dial" to the desired needle and tip it out of the case. I think I still have a somewhat similar one somewhere in all my sewing paraphernalia.
I own a 1927 singer 66 treadle. I never want to make mine motorized. If anything i need is a better sewing light. I maintain my machine myself. And mine has a temperment. It wants singer accesories and only singer accessories.
