On Libertad (Maramu #121) the zincs are attached with bolts that screw into a threaded sleeve in the rudder. If the zinc is gone it makes sense that the bolt would be gone too. Is there a hole that can be detected where the zinc should be mounted? If so, it probably needs to be cleaned out to reveal the threaded sleeve. If the threaded sleeve is gone, it shouldn't be too hard to glass in a new SS threaded sleeve to receive the proper size SS bolt.