Yes, looking at it, I agree that it probably IS the Festoon Necklace but....
According to Hugh Roberts The Queen's Diamonds
"The necklace was commissioned by King George VI for Princess Elizabeth in 1950, using a total of 105 loose collets that had passed to the King as heirlooms of the Crown"
Roberts make no mention of it ever being worn by anyone other than Princess Elizabeth (later QEII).
I guess that I am surprised to see QEQM wearing the necklace if it was supposedly a piece made specifically for her daughter (at least according to Roberts sources).