Helen's post with the dates prompted me to do some more searching in the newspapers of the time.
I was intrigued to find that the smaller Cullinans (if I can use that term for stones which included the Cullinan III and IV, which are very large by any standards) were formally presented to Queen Mary in settings said to have been chosen by her.
From the Times 29 June 1910 page 13
In The Queen's Diamonds Roberts says that the various brooches made from these stones were made in 1911. On the other hand he has a photo on page 163 for which a date of 1910 is given. It is this photo - the original negative is, I believe, held by the V & A in the Lafayette collection. (For some reason or other, I have not been able to search the V & A tonight to verify this and the date)
If the dating of this photo (1910) is correct, it suggests that, at least, Cullinan III and IV were presented in the setting we are familiar with. It would also seem to contradict Roberts' dating of 1911 for the making of this particular brooch. [Note. Roberts does state that no bill for the making of this brooch has come to light. Perhaps he was not searching under the name of the government or an official from South Africa]
I was also taken by the heading in the Times article "Gift of Crown Jewels" As far as I am aware, the "smaller" stones from the Cullinan presented to Queen Mary in 1910 are not Crown Jewels - in the same sense as the Imperial State Crown. Rather, I suspect that Queen Mary was suggesting that they would remain as heirlooms of the Crown, passing from monarch to monarch in the same way as those jewels Queen Victoria designated as jewels belonging to the Crown to be worn by Queens of the UK.
Below is a quote, as published in the Times of 29 June 1910 page 13, of Queen Mary's official thanks to the government of South Africa.
It would be a fascinating avenue to explore as I think that it has been accepted the diamonds in the various brooches from this gift are the personal possession of HM, QEII.