Dear Mr. Livesey,
  Thank you very much for your letter. The difficulty with the explanation you propose is that following the sentence in question, 'If Gandalf is only looking for Frodo, Trotter will have to be an old associate', my father wrote: 'Thus if a Hobbit, make him one who went off under Gandalf's influence.' This seems to me to show that he meant that Trotter would have to be an old associate of Gandalf's, not, as you suggest, Frodo's.
  In haste,
  Yours sincerely
  Christopher Tolkien