Early in the episode, Jo visits Amy at her farm. She introduces herself as "Senior Constable" however her jacket contains the epaulets of a Constable (no stripes). Being a Senior Constable, it is highly unlikely that Jo would not be wearing the epaulets reflecting her higher rank, especially as she is shown receiving them some episodes earlier. Further, immediately after this scene, she and Jonesy are shown in the police car traveling back from Amy's farm and you can clearly see the stripes on her epaulets that mark her as a Senior Constable.