'Tillie' Gilchrist, JW's model, may have been Mrs Matilda Gilchrist (b. 1828 or 1829), the mother of the dancer and actress Connie Gilchrist.
Tillie Gilchrist posed for JW on a number of occasions and appears in Study for 'Tillie: A Model' (M.369), Tillie: Study in Pink and Mauve (M.370) and Tillie: a model (K.117). She was a widow at the time of the 1881 census.
MacDonald, Margaret F., James McNeill Whistler. Drawings, Pastels and Watercolours. A Catalogue Raisonné, New Haven and London, 1995.