Henri-Martin Prévost is listed as a gilder at the Sèvres factory from 1757-97.
Pierre-Joseph Lorthior (1733-1813) was made engraver to the King in 1776. The portrait may represent the Comte d'Artois rather than his brother the king.
For a similar plaque inset with a biscuit portrait of Louis XVI in the collection of Stavros Niarchos, see D. Cooper, Great Private Collections, New York, 1963, p. 198.