In an outdoor daytime scene about mid-way through this episode, you can clearly see it was filmed at the Forty Acres backlot since you can see the Mayberry Courthouse from the The Andy Griffith Show (1960) next door to the market.
The title of this episode comes from the Christian bible's Book of Genesis 4:12, wherein Cain was physically marked on his forehead as a sign that he killed his brother Abel.
French title: "Le signe de Caïn".