My guess is that Garrard were making the necklace - and its case - but the work was not ready in time for the birthday - or not ready in time for photos to issue to the press ahead of the date? We are still not sure just what was handed to the Princess on her birthday.
[The captions in this first pic do not fit what we see. Just which gift is the vertical layout? ] Edit: the vertical shows the 17 diamond gift for Princess Margaret, I believe, which would be long enough for a necklace and too long for a bracelet. it is amusing how the baguettes are ignored in the stories as though they are worthless!!!!!!!!
Edit 2: more and more I am leaning towards thinking that all the diamond gifts to the 4 royals were unset. That seems to have been the preference of the givers. And the presentation cases would be a standard product of de Beers or whoever. The cases do not have to be by Garrard.