It's Christmas Mass time in the church scene, but the priest wears black vestments, which are proper in the Catholic Church only for funerals.
Prescott's mother writes the French address on an envelope with a modern five-digit postcode. But France has had postcodes only since 1964, and the current five-digit ones only since 1972.
In his first scene, the boy's father regrets that his son is "acting out." This pop-psychology expression would not exist until at least fifty years later.