There's a 15-88 on CL here.
https://dallas.craigslist.org/sdf/atq/6149512629.html (I don't plan to buy it.) It's advertised as having an oscillating shuttle. I looked it up and from the photos, it looks like the bobbin area of a 15-91. In fact, it looks like a 15-91. Does it use the same bobbin case and bobbins?
bkay