Robin Catalogue:
Carus, Paul (Open Court Publishing Co.). One hundred and twelve letters (Carus, E. C. Hegeler, T. J. McCornlack, Lydia Robinson, M. A. Sachsteder, F. Sigrist) to CSP, July 2, 1890 - September 10, 1913; twenty-five letter drafts from, July 5, 1892 - June 13, 1911, and n.d.

From Collected Papers, Cross-reference bibliography:
Two letters [to Paul Carus] in Widener VB2a. One of these letters bears no
date but was probably written c.1910. [CP] 8.214 238 except 214n1 and
225n10 are from it. The other letter is dated 19 July 1910. [CP]
8.214n1 contains a quotation from it.