I am coming to the view that Queen Mary experimented with different configurations in the centre of the tiara as well as with the surmounts, so for example she used emeralds, Cullinan III or possibly none.
Until I saw this image, I believed that QEQM was the first to wear this tiara without any surmounts.
Queen Mary. Image is from the National Portrait Gallery. It is undated but I suspect it is from the 1920s.
One question relating to this image is - is HM wearing a diamond between the scrolls in the upper portion of the centre? That is where she would normally have the Cullinan III or an emerald?