I highly doubt the Spanish were that exact when it came to making cannon balls, but if it was used in a cannon, then the weight of the cannon would have been for iron cannon balls and so you would expect a lead cannon ball of the same diameter to be heavier. Even so, lead cannon balls would be problematic and would become misshapen during transport, which would cause problems in the cannon. That is why cannon balls were usually not lead, thought shot can be. At 15.5 lb would be a large single piece of shot, LOL.