The OPD, a part of the filling valve, serves as a safety shut-off and prevents overfilling of the tank, which might lead to fire or other safety hazards. According to Diana Kees of the N.C ...

However, each state is responsible for enacting a law adopting the tank refill requirement. Earlier this year, Massachusetts and Maine made it illegal to have a propane tank without an OPD valve.

Any propane tank made after September 1998 has an OPD. If you don’t know when yours was made, just check the valve. If it’s triangular, the tank has an OPD. If it’s cylindrical or star ...

Tanks made after 1998 also have an "OPD" stamp on the valve, and the date of manufacture is usually stamped onto the top of the propane tank.