The National Portrait Gallery (see link) dates that sitting with Dorothy Wilding to January 12, 1937. Unless that was the first time the fringe tiara was used, it is possible that it was worn in the previous year and could be the sixth tiara referred to by Field.
Roberts writes that the tiara was given in August 1936. I haven't been able to connect that date to any significant event which would warrant such a gift from Queen Mary. The abdication crisis was soon to begin but perhaps it was already a foregone conclusion within the family.