On a recent visit to New York I looked at some of the manuscripts at the Brooklyn Museum. Among the more interesting ones is MS 19.74, with decorative motifs free-floating in the margins, in the manner of manuscripts illuminated by the Master of the David Scenes , such as these, accompanying the gospel extracts at the beginning of the volume:
The second of these images shows some circular brown marks towards the inner margin. I imagined that they might have been caused by something like pilgrim badges, but did not give them serious thought until I got to the suffrages at the end of the volume:
a previous post.
For someone who needed glasses to read, and who probably owned only one book, it made absolute sense to use his/her pair of glasses as a bookmark, so that they would never forget where they had left their glasses and would not lose the place they had reached in their reading.
 The Brooklyn manuscript is described in A.M. As-Vijvers, Re-Making the Margin The Master of the David Scenes and Flemish Manuscript Painting around 1500 (Turnhout, 2013), pp.515-6, 564-6, figs.172-4, and elsewhere.