The argument that anvil rebound is meaningless because the hammer energy goes all into deforming the hot iron and therefore there is no rebound of the hammer is a nonsense argument. The hammer is only half of the equation when forging.  The rebound with a ball bearing when testing an anvil is a TEST. This test is to indicate how much of the force applied to the anvil will be sent back into THE WORK, and how much will be lost into the anvil. The bigger the loss, the less efficient will your