Liver, no...not in "real life" (though in pictures, yes).
Sako is a silver sable (and belonged to a friend) so I have seen the reverse mask "in real life". He is 100% GSD.
Untitled by
jsnail17, on Flickr
I think the liver coloring in your pup is very interesting. I love sables in general
Where did she come from? You generally won't see sables like yours or like Sako because reputable breeders won't breed for those markings. Still, they are quite stunning to see!