Today I re-read parts of of Roberts, The Queen's Diamonds relating to Queen Mary's pearl jewels. As a result, I now think that the pearl and diamond stomacher under discussion did come from Princess Mary, Duchess of Gloucester.
The Duchess of Gloucester's will mentions only a pearl and diamond stomacher in a blue velvet case. [As I mentioned above]
Excerpt from the will of the Duchess of Gloucester.
However, on page 182 Roberts says that some of the pear shaped pearls used in the Lover's Knot Tiara came from the pearl and diamond stomacher left to the Duchess of Teck by Princess Mary, Duchess of Gloucester.
Significantly, in footnote 3 on page 182 Roberts cites the jewel inventory of the Duchess of Teck which details "A Pearl and Diamond stomacher of 15 Pearls", indicating that this is the bequest of the Duchess of Gloucester.
The photos which Malluu posted above show that the pearl and diamond stomacher worn by Queen Mary does have 15 pearls.
I am taking the liberty of re-posting 2 of Malluu's photos to highlight the 15 pearls as they are the best I have seen.
Malluu's images
At least two of Malluu's photo were taken after the Lover's Knot tiara was made in 1913; however, diagrams published in the Mega Munn show that some 26 of the pendant pearls originally in the tiara were detachable (confirmed by Roberts). With so many pear shaped pearls being detachable, it meant that Queen Mary could use them in other jewels and obviously this diamond and pearl stomacher was one of those jewels.
I wonder if the stomacher still exists?
If any different information is suggested by the will of Augusta, Duchess of Cambridge, I will pass on any relevant information but, for now, I am inclined to accept what Roberts says.
Many thanks David G for suggesting this line of inquiry- you were correct! Congratulations!
My thanks also to Malluu for the splendid photos which have been invaluable.
PS If any one is wondering, the handwriting in the excerpt from the will of the Duchess of Gloucester is that of a clerk in the Prerogative Court of Canterbury which then handled wills.