I am especially intrigued because it would appear the Queen Mary is wearing this (probably unidentified choker) in the portrait which Roberts has published on page 208 of The Queen's Diamonds. Roberts dates the portrait by Robert Birley to 1934. So, it could not be the completed chain link bracelets/choker of Queen Mary, as according to Roberts the second bracelet was not made until 1935.
For what my opinion is worth, based on visual observations, I feel that Arthur has correctly identified, at the very least, part of the choker, but whether there were two such bracelets as worn by QEQM is something I know nothing about. What a mystery we have!