MLA

The Centinel at His Post, or, Boneys Peep into Walmer Castle!! [Graphic]. Pubd. Oct. 22, 1803, by S.W. Fores, Piccadilly, No. 50, 0AD. https://collections.library.yale.edu/catalog/15813681.

APA, 6th edition

The Centinel at his post, or, Boneys peep into Walmer castle!! [graphic]. (0 C.E.). [Caricatures and cartoons, Satires (Visual works) England 1803, Etchings England London 1803, Watermarks (Paper) J. Ruse]. Pubd. Oct. 22, 1803, by S.W. Fores, Piccadilly, No. 50. https://collections.library.yale.edu/catalog/15813681.