Sources: UkCU
- Samuel L. Clemens to Richard M. Milnes (Lord Houghton), 1 September 1875 · Newport, R.I · UCCL11620
- Samuel L. Clemens to Richard M. Milnes (Lord Houghton), 18 October 1875 · Hartford, Conn · UCCL11621
- Samuel L. Clemens to Richard M. Milnes (Lord Houghton), 29 October 1875 · Hartford, Conn · UCCL11622
- SLC to Frederic W. H. Myers, 12 January 1892 · Berlin, Germany · UCCL11626
- SLC to Francis D. Millet, 29 September 1893 · New York, N.Y · UCCL11623
- SLC to Frederic W. H. Myers, 14 November 1896 · London, England · UCCL11624
- SLC to F. W. H. Myers, 2 July 1900 · London, England · UCCL11625
- SLC per Ralph W. Ashcroft to Mrs. Frederick W. H. Myers, 20 June 1907 · London, England · UCCL12211
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