For a long time I thought that this was an example of a particularly large medieval buckle. However, I am reliably informed by expert Brian Read that it is in fact part of a medieval book-clasp, and dates from around 1300-1400.

Books were both rare and valuable, and only among the clergy (and, to a lesser degree, the aristocracy) would literacy have been found at the time this clasp was being used.

The left side was riveted onto a leather strap, and the right side hooked onto its opposite half (on the other end of the strap), thus keeping the book safely closed. Or not, in this case!