What I noticed over the years on mopars is , Balancers on stick cars were solid also heavier , The auto cars were hollowed out in the back and lighter. This was done for inertia for pulling out with a stick car so its heavier. Automatics were lighter for torque reasons.
I am not saying this for six cars but on older V8's I see this all the time. I never payed much attention to the six's but it could be the same.,