To me, this looks very much like the top part of a baroque book clasp. Remnants of the lower metal strip are still attached to a rivet. A comparable piece can be found here: Open link: www.abebooks.de Click on the picture to see a photo of the back.
Oh, it actually looks similar. I've already measured it and got it ready for shipping. I wasn't really sure about the age, whether it might be 17th century or even older, but my gut feeling was right again ❤️👍. It's actually my first one in this shape.