Q. Hi, can you tell me what “pl.” stands for in “vol. 5 (1822), pl. 57”? Thanks!
A. It most likely stands for “plate”—as in an illustration printed on special paper and bound together in a separate section known as a gallery; these pages typically aren’t paginated with the rest of the book, so plate or figure numbers must be used instead of page numbers to refer to individual pages in the gallery. CMOS 10.42 includes nearly 250 abbreviations that might appear in scholarly publications, including “pl.” As you will see in that list, the abbreviation “pl.” can mean either “plate” or “plural” but is best avoided for the former. Whoever recorded “pl.” in the example you cite apparently didn’t see our list.