Sources: ICU
- Samuel L. Clemens to Unidentified, 13 February 1871 · Buffalo, N.Y · UCCL00574 ICU
- Samuel L. Clemens to Osborn H. Oldroyd, 18 February 1874 · Hartford, Conn · UCCL01050 ICU
- SLC per Telegraph Operator to Associated Press, 15 July 1907 · SS Minnetonka · UCCL12958 ICU
- Stephen Leacock to SLC, 7 June · UCLC46642 ICU
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