I doubt there is any SS in your hen. It is not all that uncommon for individual birds to loose some pigmentation in random feathers. This use to be quite common in birds about 100 years ago, even in show lines. Usually pigment loss occurs after they are older, such as was the case in your hen. We don't see much of it anymore as it was mostly related to nutrition. Not that the birds weren't getting enough nutrition, but that some individuals didn't seem to absorb certain nutrients as they aged and thus the lack of pigment on feathers. I've seen pictures of purebred Black Australorps that turned almost solid white in college experiments from the 1930-1940s.
You can google it and find lots of information concerning it.
All that being said, your hen is beautiful and it would be neat if one could establish a line that actually bred pure for a coloration like your hen.