Thanks for the correction, Arthur. I meant to write that the tiara is now with the Hesse-Cassel branch, who heads the "Hessische Hausstiftung". But due to lack of concentration somehow "Darmstadt" slipped in instead.
I wonder if Princess Margaret has also worn the strawberry leaf tiara. It must have been available to her. Just pictures of her are very scarce and there might never be proof of my theory.