Thinking back on these letters of Sheila's,Jeremy could easily have said " yes,it's Sheila's writing ",but he didn't,instead,when shown them a couple or so years ago,he'd said not and more or less,unknowingly, dismissed the fact that they could have helped towards his release.
Simply because he didn't recognise that her writing would have altered along with her mental state,something that he didn't give a thought to,so he was being HONEST in his refusal to accept that the writing was that of Sheila's.
I sincerely hope that all goes well in securing him part of the freedom he so deserves. A few steps forward is better than none at all.