When the pathologist is examining the man who has been shot in the car he points out the scorch marks that are left on the victims head. He says that the gun must have been no more that two inches away. The metric system in France had been in use for around two hundred years by this time.
It is doubtful the quality of the music the character Carl listen to in his house comes out of a real gramophone.
In the garage at time 1:12min there is an engine with an alternator. The time period would have engines that used generators.
Carl and Else Anderson are arrested after arriving by train in Paris at the Gare D'Orsay. The Gare D'Orsay was closed in 1939. However, the story was published in 1931.
Having a Jewish jeweller operating in an established Jewish quarter of Antwerp, including Hasidic elements of its population, would simply not have been possible in the early post-war era.
Some of the clothing styles and the motor vehicles may be post-war, in which case the depiction of period is ambiguous.
Madame Michonnet goes to light a primus stove, but clearly has no idea how to do it. She applies a match to where the meths should be, but hasn't put any meths there. Then she shakes it and decides it's out of paraffin, even though you can hear some sloshing about.