MLA
Huntingdon, Selina Hastings, Countess of, 1707-1791. Letter from Selina Hastings Huntingdon to Hill, undated. between 1744 and 1791. Selina Hastings Letters, 1774-1791, undated. Pitts Theology Library. https://digital.library.emory.edu/purl/781jh9w1fd-cor.
APA, 6th edition
Huntingdon, Selina Hastings, Countess of, 1707-1791. (between 1744 and 1791)[Letter from Selina Hastings Huntingdon to Hill, undated]. Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/781jh9w1fd-cor.
Chicago
Huntingdon, Selina Hastings, Countess of, 1707-1791, Letter from Selina Hastings Huntingdon to Hill, undated, between 1744 and 1791, Selina Hastings Letters, 1774-1791, undated, Pitts Theology Library. https://digital.library.emory.edu/purl/781jh9w1fd-cor.