Sorry, Nellie, but I do not think it is the Koh-i-Noor mounted in the centre of the Durbar tiara. We can spot the characteristical angles and the square shape of the cushion-shaped Cullinan IV (whereas the Koh-i-Noor is a regular oval).
On the 1911 picture, the Cullinan IV was positioned horizontally. After 1912, the Cullinan IV has been rotated by 45° (what I called "diagonally" in my previous message). Hence the difference of look between the two versions of the tiara. But it is definitely one and the same diamond mounted in the centre of the tiara.