This now reinforces my thoughts that the higher numbered Cullinans, given to Queen Mary in 1910, were reset - most likely by Carringtons for Garrards - as the brooches we know today, for the Durbar stomacher in 1911.
Edit: reading further, another workshop was involved in the special settings for Cs I and II to be worn as a brooch, and similarly for Cs III and IV as a brooch, and "certainly executed an elaborate tiered brooch from some of the other Cullinan stones for Queen Mary, the commision being passed on to Garrard's." More tomorrow!
Edit 2: "a London workshop acting as outworkers for Garrard & Company"