Our combined experienced has just solved it:
Some 15s do not have it; they use the bobbin winding business such as what is at the lower right of my machine.
However, some 15s do have it; they appear (by foufy's diagram) to be the kind that have an older-fashioned bobbin winder, mechanism remnant of the long shuttle bobbin winders. For these, the diagram shown, with the extra plate notch, all adds up.
The clone probably just cloned this notch for aesthetics. And the Singers which have it, that is probably because they just used up stock plates even when the notch became redundant. That is just me being a sleuth. What do you guys think?
(And Joe gets the best of both eras, by using both guides for spool wrapping
)
Foufymaus, great diagram.
Christy, When I run my stitcher, the L shaped bar to which you refer seems not to serve a tension purpose, but rather as a stop-bar for the tensioning spring so that it only goes up so far. Keeping the thread behind it and out of the way of the flapping tension spring makes the most natural sense as I watch it work, and if I put it in front, it does not make sense. The machine stitches beautifully this way. However, if there is something I am missing, please correct me; I am no expert!