Magnetic clasp. Powerful. Inside diameter is 3.6 x 28 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!
Made in Germany. 22mm x 31mm x 7mm By the each.