A few links relating to swan marks.
Example swan marks (A History of British Birds, 1843)
Guardian article on 2022 book auction
MacGregor, A., 1995. Swan rolls and beak markings. Husbandry, exploitation and regulation of Cygnus olor in England, c. 1100-1900. Anthropozoologica. 2: 39-68.
British Library: Collection of swan marks, late 16th century
Two Books of Swan-Marks, on Vellum, from Horace Walpole's library