you mentioned it being "hard" rubber. One thing many do not either know or realize is when some rubber and foam products are submerged in or have extended exposure to water the material will become more dense and even heavier. Ive handled many items from the Pacific and much of the rubber items show this. As long as the items are kept dry and in a non damp location, they should be fine. If your trying to find the material they are made from, dont be too upset if the density of the material isnt a match as per mentioned.