Alexandre-Jean-Baptiste Piochard D'Arblay (1748-1818)
"From the original crayon drawing in the possession of F. Leverton Harris" (Hill xii)
Source: Hill, facing p. 54
The National Portrait Gallery's photogravure by Emery Walker (dated 1905, plate size 5 1/2 in. x 4 1/8 in.) is also "after an unknown artist," and very similar.
D'Arblay was one of the French émigrés who sought refuge in Surrey after the French Revolution, and met his future wife, Frances Burney, there.
Image scan, text and formatting by Jacqueline Banerjee. [Click on image to enlarge.]