Because photos of Mary, Duchess of Beaufort in grand jewels are rare it is difficult to find one where the details of the stomacher are clear.
To my eye the jewel in the photo Janet posted and the one worn by the Duchess of Beaufort are the same, but it is difficult to say for certain.
The best photo I have found of the Duchess wearing this jewel is of her and the Duke with her uncle, the Earl of Athlone, at the opening of parliament in 1938.
A detail from the photo
What do others think?
The photo Janet posted again