Ah, I see! Thanks for clarifying.
Personally, I would not have known what "S" or "O" meant from looking at it on the image nor in-game, and I don't think I've ever seen those images used for those buttons before, typically I see buttons tilted left or right respectively and sometimes with the Select/Share or Start/Options icon inside or directly above. That said, my experience is only one of many, so maybe my understanding is outdated or in the minority!
Like I said, I'm not sure I'll even need these buttons for my current projects as they shouldn't be remapped I guess, but if you decide to add anything additional in the future, I'll continue to pass money your way because I love the style.
Thanks!