Three different crowns were made for the coronations of Queens Alexandra, Mary, and Elizabeth (QM)
The frame only with crystals still exists of Queen Alexandra's. It was designed to sit on top of the head. The crowns of Queen Mary and Queen Elizabeth still exist in the Crown Jewels display in the Tower of London.
There is a certain amount that has been reliably published about the making of all three crowns. The "Rundell" has never been mentioned in that literature.
Queen Mary's Crown - Queen Elizabeth's Crown Two pics of Queen Alexandra's Crown