Thank you, Frank and John. I appreciate knowing that the cannonballs you have seen don't have markings. I have never seen one. My scale is really bad, but it weighed between 15 and 16 lbs on it (doubt that is accurate, though.)

After looking up "hammer throw" on the Internet, I would agree that a cannonball looks similar to the weights used in this sport... heavy, round balls. Mine must be pretty old, though, with all of the rust and pitting on it! Thanks, again for your information and guidance... and dashing my hopes of finding a piece of U.S. history :( !