Magnetic clasp. Powerful. Inside diameter is 4 x 34 mm. Insert ... something wide?
Sometimes the magnet is so powerful, it pulls the magnet out of the other half. In this case, apply some glue and re-insert the magnet into it's slot, and wait for the glue to cure before using again!
By the way - 4 mm cube beads fit in the gap perfectly. Make a square stitch bracelet with 4 mm cubes and glue into the cavity.
Made in Germany. 18mm x 38mm By the each.