Letter dated 8.8.1806 from Thorvaldsen to Schubart (possibly mentioned here).
Letter dated 11.8.1810, from von der Recke to Thorvaldsen (requests that the bust is sent to Gotha).
Other references
Else Kai Sass: Thorvaldsens Portrætbuster, Copenhagen 1963-65, Vol. I, p. 130 (the bust was offered to Thorvaldsens Museum 1925 and 1929, but the offer was declined).