There is a picture of Rosie the Riveter over Miss Prout's desk. Rosie the Riveter didn't make her first appearance until 1943, 14 years after this show is set.
The opening shot includes books by a clock. Two of the visible titles are Wednesday Adventure by John Pudney and Otley Pursued by Martin Waddell, which were first published in 1954 and 1967, respectively. The series is set in the 1920's.
Jack and Phryne examine a carbon one-time-use typewriter ribbon to discover what was typed. This kind of ribbon was not invented until 1937.