A number of years ago, I read that Andrea Reynolds (a friend of Claus von Bulow) was married to the Hon. Shaun Plunket, childhood friend of the queen and member of the family to whom this tiara belonged.
I read that the Plunkets owned a bed and breakfast in New England, so I emailed them with a copy of the photo we had available and humbly begged some assistance.
Mrs. Plunket kindly responded with the full story: it was indeed their family's tiara; the queen's tiara had somehow broken that night en route to the event; Lord Plunket (Shaun's older brother) lived around the corner and dashed home to retrieve his own tiara for her to wear. Voilą.