Of the three Girards working at the factory up to 1817, none apparently had the initial S although the Christian name of one (active at Sèvres between 1772 and 1817) is unknown.
The sitter, Marie-Thérèse-Charlotte (1778-1851), was the only child to survive Louis XVI and Marie Antoinette. She was not quite 13 when she was imprisonned in La Prison du Temple. Marie-Thérèse, the Austrian Empress, negotiated her release in exchange for five Frenchmen. In 1799 she married her cousin, Louis-Antoine de Bourbon, duc d'Angoulême. Both she and her husband lived an unsettled life, their time spent in France thrice cut short by exile. They had no children.
The Crown and Imperial Jewels were given to Louis XVIII on his return to Paris in 1814, and he immediately sent some of them to Paul-Nicolas Ménière (the Crown Jeweller to Louis XVI) so that the stones could be used to create new jewellery for the use of the duchesse. The first finished pieces were delivered on 24th August 1814. On Napoleon's return from Elba in 1815, the duchesse fled into exile in early April, taking with her 'two rows of settings, one garnished with pendant stones' but entrusting the diadem and other jewels to the jeweller Gilbert (who later returned these to the reinstated Imperial administration). After his return to France in August 1815, the Crown Jewels were again returned to Louis. Consequently, this plaque must have been painted at some point between late August 1814 and March 1815, or from August 1815 to 1817. See Bernard Morel, The French Crown Jewels (Antwerp 1988), pps. 295-303.