MLA

Backus, Isaac, 1724-1806. A few things concerning marriage. Box 2, folder 4, item 2629211. 1748–1749. https://collections.library.yale.edu/catalog/33524477.

APA, 6th edition

Backus, Isaac, 1724-1806. (["Box 2, folder 4, item 2629211"])(1748–1749)[A few things concerning marriage]. https://collections.library.yale.edu/catalog/33524477.