Herbert Pratt

Herbert Pratt is mentioned in Leon Edel's acclaimed Henry James biography as a friend of the latter's elder brother William's at Cambridge, and seems to have been a carefree, guitar-playing wanderer who visited Henry in Italy, inspiring James to use him as a model for Gabriel Nash in The Tragic Muse (1890). Can anyone point out where I can locate this Herbert Pratt in the family tree, bearing in mind that Charles Pratt's great-grandson Andy Pratt (the singer-songwriter known for "Avenging Annie") has confirmed that HP is indeed his ancestor? Herbert Lee Pratt seems to be too young to have met James.