The knocker depicts the head of a bearded man supported by a pair of dolphins who bite his ears. The tails of the dolphins intertwine and end up in the open mouths of two other masks seen in profile.
This type of door decorations are common for Venetian Renaissance (see the monumental versions of Alessandro Vittoria). In this case some similar models can be traced in the production of the Veronese Giuseppe de Levis.
Literature: Charles Avery, Joseph de Levis and Company, Philip Wilson Publishers, London, 2016, p. 27, fig. 28.
Matteo Ceriana- Victoria Avery, The Artistic Bronze Industry of the Renaissance in Venice and Northern Italy, Scripta Edizioni, Verona 2008, pp.250-51.