Source: Bartolomeo de' Zanni, Deche (The Life) Di Tito Livio, published in 1502
This is an example of a wire watermark. The image, being located underneath heavily printed text on both sides of the sheet of paper, is difficult to identify. The image below is of the same image at a higher degree of contrast and upside down. Looking at the watermark in this way, it appears to be the image of a crown above some other image, perhaps a letter, all inside a circle.
Additional Book Information
Deche Di Tito Livio Vulgare Historiate. (Edited by) Bartolomeo de' Zanni. (Title page missing along with 28 other pages) (According to documentation, this edition was printed Sept. 16, 1502.) (Published at) Venice: Bartolomeo de' Zanni. 1502