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 ...

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.

The OPD device costs about $30. A new new tank with the device already in it costs about the same amount. The OPD valve handle has three sides instead of five or more, like on older-models.