OmerH's photos below show that the tiara was originally a complete circle https://mb.boardhost.com/historyroyaljewels/msg/1565204131.html

I have read that this tiara was made by Boucheron. I don't have access to a copy of Vincent Meylan's book; however, I wonder if he discussed this tiara and provided any details?

I am interested in knowing if the tiara was originally meant to be worn in two ways -- as a circlet and as a more conventionally sized tiara?

It looks as if by at least 1937 the then Duchess of Marlborough wore it in a more conventional tiara form.

1937 Opening of Parliament