Three elements? Do you see a cemented doublet in front and two air spaced elements in the rear? If so, it's definitely a Petzval. Did you measure the focal length? If the diameter of the glass is about 3", that would put it around 12-13" assuming a classic f/3.8 Petzval. So it was probably meant to cover whole plate. With the original metal lens cap--sweet! Is it your lens now?