Combining that info with the letter that TheRoyalsAndI posted, between the Earl of Athlone and the Countess of Tankerville, gives rise to the possible line of ownership being Olivia Bennett, Countess of Tankerville (daughter of George Montagu, 6th Duke of Manchester and wife of Charles Bennet, 6th Earl of Tankerville) then to her daughter Ida Ramsay, Countess of Dalhousie (wife of John Ramsay, 13th Earl of Dalhousie). This makes since because Ida was her only surviving child other than the 7th Earl, who she apparently did not get along with. It gets a bit sketchy from there as the tiara would have gone to Ida's third son, Alexander Ramsay (wife of Princess Patricia of Connaught) then to Patricia's great nephew, King Carl XVI Gustaf of Sweden. That last jump is the biggest because it would be unusual for the tiara to go to her great-nephew instead of the couple's own son. Of course, at any point the family could have put the tiara up for sale and Carl Gustaf saw an opportunity to buy a family tiara for his wife.
Here is some of the letter dated 27 March 1911. That is of course after Edward VII's coronation when the above pictures of Leonora were taken but before George V's coronation. I wonder if the family jewels were all in the possession of the Dowager Countess of Tankerville, what did Leonora wear to the 1911 coronation? Does anyone have any pictures?
Athlone writes, in part: "… One of the many schemes I have in hand in connection with "The Price Francis of Teck Memorial Fund" in aid of the Middlesex Hospital is the holding of a Coronation Tiara Exhibition at Messrs. Cartiers'… If only I can obtain the loan of a sufficient number of jewels … I should be most grateful to you if you will be good enough to allow the tiara, which you intend wearing at the Coronation, to be shewn …" The Countess' reply, Signed, "Lenora Tankerville," is penned on the blank sheet. She writes, in part: "…I regret being obliged to reply that although it would give me much pleasure to send anything to use for such a purpose we do not possess a single stone or article of jewelry worth showing. The family jewels “do not exist” because they were stolen by the present Dowager Lady Tankerville and formed a small part of the general plunder of this house & estate of which Her Majesty the Queen knows at least some of the facts…”"