It is the engine block from a 1948 Ford flathead V8, and was previously used by Earnest Hemingway's brother Leicester Hemingway to anchor his homebrew island micronation "New Atlantis" off the coast of Jamaica.
Here, Mr. Hemingway can be seen with his lovely wife and two children, pointing to the location of his anchor.
Look up Panope's anchor tests here and on Youtube. He has conducted the most exhaustive recreational size anchor tests in history. Further more they are not thinly disguised advertisement as many tests are.