I think we have still not solved the mystery about these earrings.
They clearly differ, by their size, from the larger ruby and diamond cluster earrings presented to Queen Mary for her 59th birthday and presented to Princess Elizabeth (later Queen Elizabeth II) as a wedding gift in 1947 (source: Field - I think it was also confirmed by the Royal Collection, but I can't find the source anymore):
(Source: 'From Her Majest's Jewel Vault' website)
They also visually differ from another pair of cushion-shaped ruby and diamond cluster earrings once worn by Queen Elizabeth II, when she presented the Queen's Medal for Music to conductor Sir Colin Davis on 8th December 2009 in London:
(Source: Chris Jackson/Getty Images Europe)
I am not sure, either, that the earrings worn by the young Queen Mary could be the ruby and diamond cluster earrings worn by the current Duchess of Gloucester (the Gloucester earrings have an added diamond stud topping the ruby and diamond cluster, but it could be a later addition):
(Source: Chris Jackson [1st picture] and Matthew Lloyd [2nd picture]/Getty Images Europe)
The possible options for the whereabouts of the earrings worn by the young Queen Mary are: 1) either they were at some time in Queen Mary's later life dismantled and their stones used to create new jewels 2) or they were gifted or bequeathed to one of the Queen's descendants (Queen-Mother Elizabeth, Queen Elizabeth II, Princess Margaret, the Gloucesters, the Kents, the Harewoods... or the Windsors!) and have remained in the vaults, or at least remained unspotted by the royal watchers 3) or they could have been gifted to one of Queen Mary's goddaughters (I don't know how many she had, but probably quite a lot). Of course, the option Nr. 1 can be combined with the options Nr. 2 and 3!