MLA

Montagu, Mrs. (Elizabeth), 1720-1800. 1 ALS to Messenger Monsey and 1 AL to Thomas Percy, 1762 Sep 11, and 1774 [no month] 17.. 1762 Sep 11. https://collections.library.yale.edu/catalog/16681984.

APA, 6th edition

Montagu, Mrs. (Elizabeth), 1720-1800. (1762 Sep 11)[1 ALS to Messenger Monsey and 1 AL to Thomas Percy, 1762 Sep 11, and 1774 [no month] 17.]. https://collections.library.yale.edu/catalog/16681984.