I'd better add some remarks. My understanding is that when Queen Mary wore the circlet of her crown (without arches) the real diamonds were restored for the occasion. But not always the Cullinans, which she wore as drops or as a brooch. Then the three replicas were inserted for the complete Crown to go back on display. Is that clearer?
I'm thinking we could have an entire thread on the occasions when the Crown and circlet were worn and identifying the major diamonds on each occasion. But I don't plan to start one.
By the way I assume it has been a replica Cullinan on the cross above the monde since 1914 as well.