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This research aims, based on an analysis of a censorship notice published on December 12, 1771, by the Royal Censorship Board, to list items that were censored and yet incorporated into the Portuguese Royal Library-now part of the collection of the Brazilian National Library Foundation (FBN). The observance of practices such as Bibliological Analysis and Bibliographic Research of rarities in specialized sources is configured as a resource for the formation and development of special collections. The contribution of the research is to promote reflection on the methods and practices employed in the identification of rare collections, in addition to further highlighting the value, royalty, and uniqueness of the Brazilian National Library Foundation's collection.