Pina and Vigier also mention this 'Sa', but unfortunately don't elaborate on its meaning - they comment " ..... some (pieces) were only signed on the bottom with a small mark, which resembles 'Sa' and can be difficult to find." It almost implies it might not have been Sa.
Certainly this mark doesn't look to be any of the known Flygsfors designer's initials.
Under this factory, Leslie Jackson gives some bibliographical info., and regarding institutions that hold collections of their work (i.e. those places where more substantial details might be found) the author includes Manchester City Art Galleries .................. you could give them a try.