The dataset "Watermarks in Incunabula printed in the Low Countries" (WILC) offers access to over 16,000 watermarks from books printed between 1450 and 1501 (incunabula) in the Low Countries (today the Netherlands and Belgium).
The WILC set comprises 16,071 illustrations of watermarks taken from the 2,229 incunabula editions preserved in 14,300 copies worldwide (many of them in more than one copy). The set includes watermarks from books held in libraries, archives and museums all over the world.
The strength of the set is the huge quantity of illustrations, all of which have detailed descriptive metadata. The addition of four geographical coordinates (longitude and latitude of the places of manufacture and preservation) allows the set to be used in various ways.
The National Library of the Netherlands (KB) offers this as open data: there are CC0 licence for both the metadata and the objects (illustrations of watermarks). Access via SRU and OAI-PMH protocols.