Lilmod,
You wrote: “The Shulchan Aruch says no such thing. He writes (75/1): “tefach meguleh b’isha b’makom shedarka l’chsoso, afilu hi ishto, assur l’kros krias shema k’negda”. Which you translated as: If a tefach is showing in a woman of the places that are supposed to be covered, a man is not allowed to say Krias Shema in front of her. But that is not what the words actually mean. “Shedarka l’chsoso” doesn’t mean “supposed to be covered” it means “that is her way to cover it.”
The Bais Yosef, in his sefer Bais Yosef explains his opinion and how he learns the Gemara. The Mishna Berurah’s mehalech in the Gemara does not fit the Bais Yosef. The Mishna Berurah is allowed to argue on the Bais Yosef, but he is certainly arguing.
According to the Bais Yosef the Gemara’s statement of “shok b’isha erva” is an example of something you might have thought was not a makom mechusa, because it was typically uncovered by men, k’mashma lan it is a makom mechusa, because women usually keep it covered. The Bais Yosef is clear that there is no separate, special or new din from shok b’isha erva. And, according to the Bais Yosef’s way of learning the Gemara, shok must mean the lower half of the leg, which men did not always cover.
On the other hand, the Mishna Berurah, l’shitoso, could learn that shok b’isha erva is referring to the top half of the leg, because it is stating a special din by shok that it always has the status of erva.