Princesse Katharina Henckel von Donnersmarck, nee Slepzow by ? (photo from Sotheby's)
She was married to German industrialist and nobleman Guido von Donnersmarck who was one of the richest men of his day.
From elogedelart.canalblog.com/tag/Duchess%20of%20Windsor.
This image is essentially impossible to date. Katharina is hardly a footnote to history, but her jewels are famous. She is as lavishly jeweled as a Russian Grand Princess in this portrait. This image is placed here by analogy to this image of Queen Maud.
Keywords: Katharina von Henckel, German, curly coiffure, tiara, sweetheart neckline, no sleeves, necklace, earrings