Draft letter dated late 1797 from Thorvaldsen to Abildgaard (from this it appears that the marble version of the bust is far advanced, so the original plaster, A208, must have been completed earlier).
Other references
Else Kai Sass: Thorvaldsens Portrætbuster, Copenhagen 1963-65, Vol. I, p. 36, 37, 40, 42, 43, 45-51.