A detail (which nonetheless appears as a small enigma to me), is why Tolkien's letter from April 1970 to Tom Shippey is given an uncertain date ("?12 (received 13)") in the J.R.R. Tolkien Companion and Guide: Chronology, p. 749.

Shippey writes (in his Preface to the Second Edition, The Road to Middle-earth): "... the letter Professor Tolkien wrote to me on 13 April 1970". It seems like I'm missing some other reference to this letter - anyone knows where I can find it? Is the letter undated, or is it perhaps Shippey's ambiguous wording which has led to the uncertain dating in the Chronology?