I found the image below of Queen Mary wearing the Delhi Durbar tiara in the Illustrated London News 20 May 1911.
Here is a slightly closer view of the tiara.
It seems to me that she is wearing the Lesser Stars of Africa mounted on the tiara; yet on page 178 of The Queen's Diamonds Roberts says, "In 1912, the centre of the Delhi Durbar Tiara was altered twice by Garrard, to take either or both of the two Lesser Stars of Africa...' In footnote 6 Roberts quotes the firm's ledgers. On 19 March 1912 "Altering centre of large emerald & diamond Tiara, mounting Your Majesty's 4 small brilliants & 17 roses as ornaments on either side of large drop brilliant...' And for 14 May 1912 "Altering centre of large emerald & diamond Tiara to take large square brilliant when emerald drops are worn..."
After looking at the photo in the newspaper I wondered if the proportions of the upper part of the centre of the tiara were altered creating a slightly different shape, although it is possible that this is a trick of the light and the angle from which the photo was taken.