MLA
Cutler[?], Arthur. Letter by Unknown Author to Their Child. 1918. Stuart A. Rose Manuscript, Archives, and Rare Book Library, Box 3, Folder 37. https://digital.library.emory.edu/purl/576rxwdctf-cor.
APA, 6th edition
Cutler[?], A. (1918). Letter by unknown author to their child (Box 3, Folder 37) [Letters (correspondence)]. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/576rxwdctf-cor.
Chicago
Cutler[?], Arthur. “Letter by Unknown Author to Their Child.” Letters (correspondence). Civil War Collection, 1918. Box 3, Folder 37. Stuart A. Rose Manuscript, Archives, and Rare Book Library. https://digital.library.emory.edu/purl/576rxwdctf-cor.