I was convinced that on all Holga's the sunny/cloudy switch was just for show, as it turns out on a GN (glass lens) Holga it does do something- it gives you two aperture settings.

Holga 120
It took me some time to figure this one out. Turnout that there’re Holga’s with a working sunny/cloudy switch and there’re Holga’s where it doesn’t change the F-stop at all. The rule to this is actually fairly simple, if you have a glass lens Holga (the GN) you really do have 2 F-stop settings, the plastic lens Holga’s have one F-stop setting and changing the little nob to cloudy or sunny won’t change a thing. 

For me at least, mystery solved :-)