On the train Miss Fisher gazes up at a crescent moon. A day or two later there is a full moon over her house.
A police sergeant is never referred to as "Sir". That honorific is used only for commissioned officers, or in the military services those who are warranted officers.
During the train scene, Eunice is reading a novel named Bandit Love by Juanita Savage which was initially published in 1931, and the series are set during the late 1920s.