I read somewhere - I think on Superior's site - that the most common bobbin is the "L" bobbin.
https://www.superiorthreads.com/medi...ility-list.pdf
It's the same diameter as the 15, but it's a little shorter.
I've noticed that a lot of companies like to "hide" what type of bobbin they're using, then jack up the price of their properly part numbered ones.
I've also found that I can use a "L" bobbin in place of a 15 bobbin in horizontal bobbin machines at the very least.
I did it in the Kenmore a few weeks ago. I used some of Lucey's prewound "L" bobbins in it, not quite by accident, mostly out of curiosity. Beautiful stitches.
Because of that, the "L" bobbin would likely be the first bobbin type I'd try in an "unknown" situation.