Not a letter. Documents .111549a-.111549e are 5 leaves of errata in what were likely proofs of the first edition of volume III of Principia Mathematica. The verso of the 5th leaf has more corrections in Whitehead's hand, all crossed out. Some of the errata were listed as such in vol. III, some were corrected in the text of the first edition, some were corrected in the second edition, and some were never corrected.

Since volume III was not published until April 1913, it is possible that these leaves date from 1913.

Some of the errata concern volumes I and II.

The leaves were removed from between pp. 14-15 and 64-5 of BR's Library copy of Principia Mathematica, vol. I (Russell's Library, no. 3069).