This was Jumping Dolphin brand NOT Black Duck! Good strong spring. The rubber coating is non-slip so it stays in place. Poor Unadvertised brand. The metal is thin and thin. Zinc is not mentioned anywhere on the packaging. Supplied in a pack of 2. 12 packs of 24 clamps. After a couple of weeks there is no sign of rust on the road. Must be galvanized. Gave an extra star.