I. Charlton is mentioned a lot in Kelly's letters (often as "I.C." or "I.Ch.", once as "old Isaac"). The most likely candidate I can find on FamilySearch is Isaac Charlton, b. Apr–Jun 1838 York, d. 4 Oct 1924 Myton-on-Swale, Yorkshire. The 1901 Census shows him as living at Altofts, a place Kelly refers to in connection with him several times.