The state openings of Parliament took place on March 12 1974 when the queen wore day dress and no jewels, another grander ceremony happened on October 29 when QEII wore the Baring Necklace. The next year she wore on November 19 the Jubilee necklace as she does in the video posted and the dress matches so I suspect that this was 75. The problem is that Georgie married Lord Colin on March 23 1974 and broke up with him nine months later which would be December and then divorced him n spring summer of 1975 the lady cannot be her as they were divorced by that time.