MLA
Boring, Jesse. Letter from Jesse Boring (New Orleans, Louisiana) to Robert Paine ([?], Alabama) - July 23, 1850. 1850. Pitts Theology Library, Box 1, Folder 4. https://digital.library.emory.edu/purl/2085hqc0sg-cor.
APA, 6th edition
Boring, J. (1850). Letter from Jesse Boring (New Orleans, Louisiana) to Robert Paine ([?], Alabama) - July 23, 1850 (Box 1, Folder 4) [Letters (correspondence)]. Pitts Theology Library. https://digital.library.emory.edu/purl/2085hqc0sg-cor.
Chicago
Boring, Jesse. “Letter from Jesse Boring (New Orleans, Louisiana) to Robert Paine ([?], Alabama) - July 23, 1850.” Letters (correspondence). Robert Paine Papers, 1828-1882, 1850. Box 1, Folder 4. Pitts Theology Library. https://digital.library.emory.edu/purl/2085hqc0sg-cor.