I suspect there was no mention of the sautoir between 1953 and Roberts' book simply because QEII chose not to wear it -- at least in public. The reasons why QEII chose not to wear it are no doubt manifold; she had her own South African necklace which has a similar design and which she shortened to her taste.
Perhaps more interestingly, Roberts has modern photographs of each jewel.
Re Queen Mary wearing the sautior as two necklaces. The photos I have found suggest that she wore the sautoir as two necklaces for day wear on semi-grand official occasions, such as garden parties at Buckingham Palace and for a lunch at the Guildhall.
Garden party 1937
Garden Party 1938. Only one necklace was worn.
1939. Lunch at Guildhall for a ceremonial welcome home for George VI & QEQM after their north American tour.
Queen Mary wearing the sautoir for evening and formal portraits.
The back of the necklace from the same occasion as above.
A formal portrait in 1930. This photo was found by J-F.