The "A" sticker seen on Jim-Bob's windshield is a gas ration sticker. During World War II, the stickers had to be affixed to the car's windshield, depending on need. To get your classification and ration stamps, you had to certify to a local board that you needed gas and owned no more than five tires. The "A" sticker is the most common of the WW2 Gas ration stickers, and was issued to the general public.
According to Patsy Rahn, the actress who played Betsy, there was a scary incident during the filming of the biplane sequence where the engine cut out required an emergency landing. She retells the tale in her YouTube interview with Judy Norton.