joe your post came in between the time I started my reply and the time I sent it.
often on this site people call the stop motion knob and thumb screw and a few other terms. as in big round thingy.
as I ask in the start of this thread . can you turn it over easily ? if not teh upper shaft is bent , she doesn't answer that.
none of the machines I just posted are parts machines, examples of what you can get and at what price in her local area. should a person chose to repair that machine ( 15-91), using one of those is a much more logical way to source parts , opposed to using ebay or ? and paying 200 x's the price of those machines.