[Fiddlehead]

The pic is a bit blurry, but I see what you're talking about.

That position is normally used for the Air Conditioner, but it's tough to say in your case.  Tried to narrow it down by process of elimination, but couldn't read all the labels in your pic.  Didn't see another that looks like "Air Conditioner" though, so that may be it.

Some work has definitely been done in your panel, as some of the breakers are white, rather than the original black.  This isn't necessarily a problem, other than the unlabeled switch.

Good luck with your troubleshooting!