When I saw the colour image of the Queen Mother wearing an orange stone brooch in two parts posted by British Royal Jewels https://www.instagram.com/p/CWn7T_tIlyp/ I immediately recognised it as the brooch Franck has been interested in for a long time. As I noted in my post below, as I find additional images of QEQM wearing the brooch, which divides into 2 parts, I have posted the information and images.
The photographic evidence from various occasions shows that the brooch divides into 2 parts, and that QEQM occasionally wore one on her hat and one on her dress.
For a Garden Party at Buckingham Palace in 1937 QEQM wore the jewel as a pendant brooch, and in the video I posted a link to, the bottom element could be seen swinging freely.
The poor quality of the images we have of QEQM wearing this brooch/es makes it difficult to be 100% certain, but I believe that on each individual element there is one side without diamonds, which allows the two elements to be linked together -- and that this then forms the pendant to the jacinth brooch. It is noticeable that where the two elements have diamonds, that there are larger diamonds at the corners with some smaller diamonds in between. When I counted them, the number of large diamonds is the same as those discernible in the pendant to the brooch, which QEQM lent for the charity function in 1965.
Two views which suggest that one side of each element is without a diamond border.
In the 1970 colour image it seems clear that one side of each element is without diamond borders.