This object was in common use in medieval libraries, even though very few survive today. It’s a bookmark - and a smart one for that matter. As with our own bookmarks, it tells you where you are in the book: the rope was attached to the binding and placed between two pages. The reader subsequently pulled down the marker along the rope to the line where he had stopped reading. Since an open medieval book often presented four text columns, the reader then turned the disk to indicate in which column he had left off. In this case we read “4” in medieval Arabic numerals - the column on the far right. So this tiny piece of parchment marks it all: page, column and line. That’s what I call smart.
Source unknown, likely 13th or 14th century