Alexander Wilson (1766–1813) based his description of the “Slate-colored Hawk / Falco atricapillus” (=Accipiter gentilis atricapillus) on a single specimen collected near Philadelphia, Pennsylvania, which he deposited for posterity in the Philadelphia (Peale) Museum. Wilson's illustration of the specimen was engraved and hand-colored prints were published in American Ornithology vol. 6 (1812b, Pl. 52) to accompany his scientific description of the species. However, the path of Wilson's type specimens became difficult to trace after the Peale Museum closed in 1846 and its collections were sold and dispersed. Wilson's holotype of F. atricapillus was presumed lost or destroyed until November 2020, when I rediscovered it in the collection of the Academy of Natural Sciences of Drexel University (ANSP), thanks to a penciled note made by the late Rodolphe Meyer de Schauensee, former ANSP curator of birds. This research also sheds new light on the history of the Peale Museum bird collection, exposing a basic misunderstanding about the “Peale numbers” cited by Wilson, which has broad implications for the history of American ornithology. In short, “Peale numbers” were assigned to species (not specimens) held in the Peale Museum.