The tanks have always been over filled if filled beyond the 80% amount. The tanks have always had a "bleeder" valve that IF used vents liquid propane when the level hits 80%. There is a dip tube that extends down to the 80% level. If the valve is closed it is possible for the supply pump to overfill the tank. Some dealers would. That's why the OPD valve was invented; to prevent dealers from going over 80%. It's limited to 80% to keep the over pressure valve from discharging when a cylider gets heated on a hoy summer day.

At least that is my understanding.