Today I saw this image of Princess Alice, Duchess of Gloucester.
Could she be wearing the bandeau/necklace you are interested in? I am not sure, but some things remind me of the necklace.
One of the reasons why I am unsure about this is that I cannot see the links between the elements which are now in the necklace, and this jewel has a row of diamonds beneath it. Perhaps my eye has lead me into confused thinking.
The photo was taken in March 1950 for a dinner at the French Embassy during the French State Visit to the UK