Not sure yet, but I suspect the casting number tells them which balance factor the crank has drilled into, and they are different. I will try to keep this short.
I have a 92 casting block. Bought damaged. A few broken rings. Came with Edelbrock aluminum heads so no idea which way it was originally set up, but suspect it was in a 93 model year truck, making it a Magnum.
I asked a local Mopar shop to balance it as an LA when I replaced the pistons to fix it. Supplied an LA version B&M flex plate. Put it all together as an LA and it vibrates. Figured out the difference between LA and Magnum converter weights and welded the difference into the flex plate cutout area on my converter as a test, still in the car. Vibration is fixed, gone. So that engine needed a Magnum flex plate.
My theory is they balance it to a Magnum factor based on the casting number, which I think it was the same casting crank and rods as the Magnum stuff I compared to.
