I'll say openly that I think Arthur was correct in identifying the source of this choker.
I agree with him that it relates to the bracelet Queen Elizabeth the Queen Mother has been photographed wearing.
Whether there were 2 bracelets which formed the choker, is another question. What ties this, or one of these bracelets, might have to Queen Mary's chain link bracelets/choker is a subject which I think will give us a lot to think about in the future.
I am looking forward to finding more evidence to help us solve the rest of this mystery.