After quite a lot more searching I am now satisified that the choker in the portrait cannot be Queen Mary's Chain-link Bracelets described on page 210 of Roberts 2012. I believe the portrait dates from 1928.
Edit: to explain, the NPG owns the plates so their year oif 1928 has a good chance of being correct. The Royal Collection probably has a postcard and has dated it from possible time of acquisition of the card.