MLA

Buck, Daniel Webster, 1840-1892. Daniel Webster Buck letter, Port Conway, Virginia, to "Dear Sister Julia". Box 1, folder 6. 1863 February 15. https://collections.library.yale.edu/catalog/17437186.

APA, 6th edition

Buck, Daniel Webster, 1840-1892. (["Box 1, folder 6"])(1863 February 15)[Daniel Webster Buck letter, Port Conway, Virginia, to "Dear Sister Julia"]. https://collections.library.yale.edu/catalog/17437186.