The lake where the picnic occurs is in reality Upper Franklin Canyon Reservoir in Franklin Canyon Park in Los Angeles, CA. This body of water and the surrounding area has a long history in Hollywood movie making as the site for many movies and TV shows, perhaps most notably in the opening credits to The Andy Griffith Show (1960).
Fred Rutherford wears a Yankees baseball cap when the two families go on a picnic.
The picture Fred took of Violet kissing Beaver isn't the same one on the cover of the magazine. Beaver's hair and posture are different in each one.