So you're saying two of the three strands in this portrait are in fact one collet necklace, twofold and extended at the back with parts of the diamond sautoir?
As far as I can see the size of the diamonds is decreasing from the front backwards in all three strands. Wouldn't a single strand necklace arranged in two strands result in the smaller diamonds from the back showing up centre front in one of the strands?
Or do you suggest that the diamonds were rearranged according to their size in two short strands at one point or the other and permanently extended with parts of the sautoir?
Is there any other photographic evidence clearly showing parts of the sautoir as an extension at the back? Because I don't think these photos in the resolution provided by the National Portrait Gallery allow such a conclusion to be made with some certainty.