Else Kai Sass: Thorvaldsens Portrætbuster, Copenhagen 1963-65, Vol. I, p. 212-218 (There are two marble versions at Canonteign, Devon. One is very well executed and signed with Thorvaldsen’s monogram on the back (see example of Thorvaldsen’s monogram in the survey of Thorvaldsen’s works). The second marble copy is unsigned and a little less well executed, cf. Sass, op. cit. p. 216-217).
Thiele II, p. 298 (Thiele mentions that the bust was commissioned in the summer of 1816, but as it was already completed, or presumed completed, in marble at this time, this cannot be true, cf. letter dated 31.5.1816. Maybe the work was re-commissioned, cf. the two marble versions).
Letter dated 31.5.1816 to Thorvaldsen from Pellew (suggesting that the marble bust should be completed at this time).