What do last numbers in block casting mean?

legend has it as the number of the core revision. So there is maybe a minor correction in a wall thickness so the change the core to #2, and so on. Supposedly the lower core numbers were better because they were heavier, as in more metal. Later cores were pinching pennies to conserve iron. I also head they cast them 8 at a time and its just the position in the grid that they were in..