Someone placed their eyeglasses, most likely leather-framed spectacles, inside the flyleaf, closed the book, and then went off and forgot about them or got killed by Huns or something. It’s like a coffee ring, only made by leather eyeglasses.
They might have looked something like this:
Advanced scientific methods for dating aside, we can get a good estimate of the age of the eyeglasses that left the impression on the parchment by first examining the script on the parchment (to establish the earliest possible date) and then by looking at the shape of the impression itself. The text is what is known as Southern Textualis or Rotunda. Southern Textualis was popular in Italy and Southern Europe between the late 1200s and the late 1400s. Alternately, the 1568 publication of the printed text provides us with a possible later date. Regardless, the spectacles conform to the physical features and rough time period for early medieval leather-framed spectacles. But dare we hope for more? Because the book was printed in Venice, Italy, the tantalizing possibility exists that the wearer who deposited his spectacles in between the parchment leaves may have been using a pair of the earliest eyeglasses ever made, because Florence, where eyeglasses were invented, is less than 165 miles from Venice. Although we may never know exactly how (or when) these spectacles left their mark on the parchment, their faint impressions nevertheless offer an intriguing glimpse into the early history this important invention.