See de Ricci, English Book Collectors, pp. 64, 68, 102, 105, etc.
One of the main London booksellers from the 1820s until his death, who "went far to cornering the market in historical, genealogical and topographical manuscripts, and issued a number of catalogues devoted entirely to such material" (Munby, Phillipps studies, III, p. 46).
Thorpe marked books with a code, usually in pencil, in lower-case letters (which g=1 and n=3; I have not deciphered the rest of the code), in the top left corner or centre of the upper pastedown.
Here "gu/-" is Thorpe's; "A/I/2" is Kraus's, "Thorpe" is Phillipps's; and the other marks are unidentified
Here "g/ge/-" is Thorpe's; "C/VI/8" is Kraus's; "Thorpe" and "Celotti" are Phillipps's; "56" is the lot number of this manuscript in the 1825 Celotti sale; and "161" is perhaps a number written by Thorpe or Celotti.