What we need is a firm dating for the portrait of Queen Mary.
Some other interesting things which Roberts says on page 210 are
That the detachable brooch with the large diamond, which is part of the chain link bracelets/choker was made in 1934, QM having received the large diamond in 1933.
The second bracelet was made in 1935. Roberts says this was designed to incorporate the detachable brooch.
So, if Robert's dates are correct then, in the 1934 portrait of Queen Mary on page 208, her choker cannot be the completed chain link bracelets/choker. I would suspect that only the front section, which does seems to feature the detachable brooch section, is part of the chain link bracelets/choker.
If this choker does show the detachable brooch, then the question is whether this was mounted on the the bracelet purchased in 1932 or on another.