Thin pieces of metal that are bluntly attached to precious illuminated pages. It is not something you see every day in a medieval book - or imagined to see at all in such delicate objects. They are pilgrim’s badges, mementos purchased during pilgrimages to holy sites in medieval Europe. They are really not very different from the Eiffel Towers, baseball caps or Big Bens that we carry home in our suitcases today: they are mass-produced, cheap and highly portable souvenirs. If you went to see the shrine of St Thomas Becket, you would take a badge home, partly to show that you had been (like this one). The badges above are special because the pilgrim attached them to the pages of his prayerbook when he came home, which is how they survived. The shiny pieces of metal are religious instruments, of course, but they also proudly emphasize that the owner of the book went on a real pilgrimage: been there, done that!
Pics: Oxford, Bodleian Library, Douce MS 51 (Book of Hours, Flanders, c. 1490). More images and information here. More about medieval pilgrimages here. A safer (but not cheaper) alternative was to have pilgrim’s badges painted into a book (here).