I agree that this necklace is different from Queen Alexandra's necklace worn in Canada.
From left to right: New Zealand, 2002; Russian return banquet, 2003; Royal Variety Performance, 2003; Abu Dhabi, 2010; Royal Variety Performance, 2012
I find it interesting that this necklace didn't make an appearance until after the Queen Mother's death, although Roberts states that the Queen inherited two of Queen Mary's rivieres, so perhaps the timing is strictly coincidental. Has anyone seen it being worn previously?