Meter Serial optional, but mandatory within OCMF v1.3/v1.4

[ahzf]

Within OCMF the Meter Serial is mandatory: https://github.com/SAFE-eV/OCMF-Open-Charge-Metering-Format/blob/master/OCMF-en.md#meter-identification

but in https://github.com/SAFE-eV/transparenzsoftware/blob/master/src/main/java/de/safe_ev/transparenzsoftware/verification/format/ocmf/OCMFPayloadData.java it is optional:

    /**
     * Meter serial number, optional field
     */
    protected String MS;

Which of both it the ground truth?