In one scene there were beautiful clouds in the sky, but there was a jet plane contrail. It was 1940 there were no jets.
When George is driving away from Louisa Mae's farm, there are several large round bales of hay on the hill behind him. Round balers weren't invented until the 1960s.
The second submission Lou sends to Saturday Evening Post shows the envelope with a zip code for her return address. Zip codes were not in use until decades later.