One of the photos Thomas posted showed QEII's South African diamonds as originally set in a necklace. In the centre is the detachable snap piece which had the 6 carat diamond which HM had been given separately.

As the formal photos of HM wearing the necklace in the longer length were all taken from the front, I had never understood what Roberts meant by a detachable snap piece until I saw a video. In it all became clear.