MLA
Fitzgerald, O. P. (Oscar Penn), 1829-1911. Letter from O. P. Fitzgerald (Nashville, Tennessee) to Mary Eliza (Millwater) Paine - May 1, 1882. May 1, 1882. Robert Paine Papers, 1828-1882. Pitts Theology Library. https://digital.library.emory.edu/purl/552w3r23fb-cor.
APA, 6th edition
Fitzgerald, O. P. (Oscar Penn), 1829-1911. (May 1, 1882)[Letter from O. P. Fitzgerald (Nashville, Tennessee) to Mary Eliza (Millwater) Paine - May 1, 1882]. Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/552w3r23fb-cor.
Chicago
Fitzgerald, O. P. (Oscar Penn), 1829-1911, Letter from O. P. Fitzgerald (Nashville, Tennessee) to Mary Eliza (Millwater) Paine - May 1, 1882, May 1, 1882, Robert Paine Papers, 1828-1882, Pitts Theology Library. https://digital.library.emory.edu/purl/552w3r23fb-cor.