This bronze book clasp or hasp was found in England early in 1998 along the Thames foreshore (river bank area concealed at high tide, but revealed at low). It is 2 3/8 x 1 5/16 inches (58 x 33 mm), with a patina of various greens, retains remnants of two rivets with which it was attached, and is decorated with a design of pierced and stamped (or engraved) circles.
Variants of this design were used all over Europe through the fifteenth century. They became smaller in the sixteenth century before disappearing in favor of ribbon ties or no fastenings.
Typically such clasps as this were riveted to leather straps that would wrap around a book, keeping its pages flat when not in use.
This is an especially interesting object considering how very few literate people there were during these times. Of the few books that were created, most were written in Latin, very few in the vernacular. It was in the middle of the fifteenth century that Johann Gutenberg (German, c. 1400-c. 1468) devised his innovative printing process employing moveable type. Gutenberg's Mazarin Bible (c. 1455) is considered the first book printed with such type. Books printed during the first fifty years of the use of movable type, from c. 1455 until 1501, are known as incunabula (a Latin word meaning "swaddling clothes" and "cradle"). So this hinge was used in the binding of either a codex (handwritten book) or of an incunabulum, among the first of the printed ones.
From a message about this clasp from Frank Mowery of the Folger Shakespeare Library, Washington, DC.:
July 12, 1999
I am pleased you found our book Fine and Historic Bookbindings interesting. [Frederick Bearman, Nati Krivatsky, and J. Franklin Mowery, Fine and Historic Bookbindings from the Folger Shakespeare Library, 1992] I wrote the sections on the clasps and furniture for books, and I have also written a whole article on the history of clasps for the Guild of Book Workers Journal, Volume XXIX number 2, published Fall of 1991. Your part of the clasp is called the hasp, and is of a relatively common shape, but I have never seen one quite like it with the four large pierced holes; Italian clasps often have piercing, but different than yours. Yours does have a simplicity of shape and finish that makes me think that it is probably rather early (15th century?) but it is hard to date such things because they rarely changed designs for centuries, common books had common simple pieces of furniture. It could have been on any kind of book, its size though clearly indicates that it would have been on a rather large and thus most likely religious book. Again not necessarily though. Brass is the most likely material you are dealing with. Iron was used earlier, but brass was easier to work and tends to be the most common, other than silver for more precious books.