This is pure speculation on my part, but I wonder if the diamond bandeau, which featured in the 29 November 2000 auction when Princess Marina's large diamond earrings were sold, could have been the tiara from the Duke of Kent?
This bandeau breaks apart to form 3 separate ornaments.
A report in the Advocate 29 December 1934 has also made me wonder if the ruby bow brooch which was part of Queen Mary's present was designed to be the ruby centre of the bandeau?
The design of the ruby bow brooch from Queen Mary contrasts sharply with the style of gifts HM gave to her other daughters-in-law. I have never seen a photo of Princess Marina wearing the brooch.
Hopefully, Vincent Meylan's book will solve the mystery.