The find pictured here was unearthed very recently by Martin Depear. Similar finds are sometimes catalogued as buckles but they have pins whereas Martin’s find never had one.
It is made of copper-alloy with a crowned head at the top of the opening and a round bar at the bottom. One end of a leather strap or belt would have been secured to the bar. The other end would be secured by friction by the crowned head. Therefore, items like this are best described as clasp fasteners. It dates from the 14th century.
I’ve seen a number of examples of this clasp fastener as detecting finds so they aren’t particularly scarce. This one is in good condition but my price range would be £12-15 at best.