A Plaque for Mayberry - S1-E25
Visible crew/equipment: When Andy and Barney walk into Mayor Pike's office, they stand beside the two women from the Historical Society, and the faint chalk curved marks are visible on the floor where they stand.
Andy and the Gentleman Crook - S1-E21
Visible crew/equipment: When Aunt Bee is in the kitchen preparing to take a tray of food to the prisoner she momentarily walks out of the kitchen through the swinging door. As she pushes the door open, a crew member's hand and arm can be seen behind the door.
Mayberry Goes Hollywood - S1-E13
Visible crew/equipment: Near the end, after the producer tells the townspeople to get back to normal and out of their trick or treat outfits, when the crowd disperses we can see some curved chalk lines on the ground near the producer's feet, to mark actors' positions.
Visible crew/equipment: When Barney and Floyd are spying on Sam while he's getting groceries, we can see both of the grocery store's windows and its clear glass, but when Sam drives off the left display window has been covered with a matte dulling spray resulting in an uneven matte surface, to prevent the camera and crew's reflections.
Visible crew/equipment: After Barney tells Andy he overheard Dr. Benson and Ellie "talking marriage" while spying on them, Andy heads over to the drugstore. Then, when Barney enters the jail cell, we can see a crew member's fingers (at the lower right edge of the bars, behind the wall) push the cell door closed. (00:19:45)
Visible crew/equipment: At the end when Aunt Bea enters the courthouse, she doesn't close the door completely. You can see a hand from outside grab the door, and close it very slowly. (00:23:20)
Answer: As noted in the previous answers, in real life, things like this provided wind and/or rain deflection, and also maintained a bit of privacy when blinds were raised somewhat. The interior courthouse set was located in the studio, so the "outside" Main Street didn't exist. I believe these things were added to the courthouse windows for practicality, to avoid some crew movement being visible on the opposite side of those windows. These are not "window boxes" to hold anything, as they're actually bottomless; we can see the Venetian blind's long pull cords under them. They're made of plywood and simple to build, so the "material and labor" was inexpensive. Similar variations made of different materials are in other movies/shows. In 1957's "12 Angry Men," textured chicken wire glass panels are in the jury room windows, and in "Jesse Stone: Night Passage" another type is in Jesse's office windows.
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