I would bet on the second bandeau tiara with the sapphire in the center. The interesting detail about your picture is that it shows the side of the tiara. Older Belle Époque tiaras were positionned on masses of hair (false and real) and their bases were very flat. This made them much more difficult to wear on more modern hair do. The Durbar tiara is a good example of that. I am sure one of the reasons the duchess of Cornwall does not wear it very often is that it is not very safe on her head. Those bandeaux, on the opposite, seat flat on the central part of the head and then they turn on the side a bit like a ribbon. This makes them much easier and safer to wear. This might be one of the reason why queen Marg ordered a few of them in the 1930s and 1930s. They were surely more practical for less grander occasion like theatre or diner parties when you wanted to move your head a bit without risking to see your tiara falling.