I think Bianchigirll and palincss are both right, in their own way, about what Bianchigirll calls the 'faux mixte', with the single dropped top tube and twin stays - it was a direct evolution of the 'original' mixte which resulted in a more efficient but, to my eyes at least, less aesthetically appealing frame. So engineering-wise (and historically) it's absolutely the real deal but looks-wise maybe not quite so much.

Mind you, the above is splitting hairs when you look at Peter Weigle's lovely interpretation of that particular design: