Visible crew/equipment: When Otis tries to escape from Aunt Bee's clutches by hiding in the laundry service van, just as Aunt Bee has the driver open the van's back doors we can see the reflection of the moving boom mic in the left window.
Visible crew/equipment: When the squad's siren gets stuck blaring, a crowd gathers around the squad with Andy, Barney and Rogers, and when the crowd disperses we can see at least three chalk T-marks on the ground.
Visible crew/equipment: When the new manicurist tells Floyd that she works only on commission, she lifts the small table and moves it to the window, and we can see the actor's tape mark where she had just been standing.
Visible crew/equipment: At the start of the last scene, when Andy and Rogers walk out of the courthouse the reflection of a crew member is visible on the surface of the car, and as Rogers drives away we can see the reflection of the camera dolly on the car, and also the shadow of the boom mic on the ground.
Visible crew/equipment: When Rogers shows Andy the statistical chart and they get a call about the domestic violence, the two chalk marks can are visible on the floor below the bulletin board to its left, which is precisely where both Rogers and Barney have stood at different times during this scene.
Visible crew/equipment: When 'FBI Agent' Jenkins congratulates Andy and Barney on behalf of J. Edgar Hoover they're all standing near the door, but when it cuts to Jenkins turning around and they all walk over to the desk, we can see the two curved chalk lines on the floor where Jenkins' feet were positioned, as well as the curved chalk marks on the upper step where the mayor was standing.
Visible crew/equipment: After Andy and Barney walk out of Floyd's shop, when Andy makes a crack about Barney's ears being uneven Andy walks offscreen, then Barney shouts at him as Andy's presumably already down the block, but in their reflection in the window behind them we can see that as soon as Andy walks offscreen in this closeup, he actually stops and leans over where we see the camera's located.
Deputy Otis - S2-E31
Visible crew/equipment: After Otis admits to Andy the reason why his brother thinks that he works at the courthouse Otis walks back to the cell, but when Andy calls him back and gets up, just as Andy walks around the desk we can briefly see two actor's marks where they will stand momentarily.
Visible crew/equipment: When Barney and Thelma Lou leave Mary's house the curved chalk actor's mark is visible on the floor by the door. Then Barney shows up again with his drum, and we can see a chalk T-mark on the floor. That same T-mark is also there in a later scene, when Barney barges in with the letter he found behind the file cabinet.
Visible crew/equipment: Right after Mrs. Williamson leaves the barbershop upset about the carp killing, when the mayor says that they have more important things to talk about - which is the celebration, we can see the curved chalk actor's marks where Williamson had been standing.
Visible crew/equipment: When Barney's reading the Sunday comic strip Andy comes in to close the jail in the courthouse, and when Otis walks out of his cell we can see the actor's chalk mark on the floor where he stands.
Visible crew/equipment: When Barney and Andy are waiting for Virgil to arrive, just as the bus pulls up and stops, the reflection of the boom pole/mic is visible on the windshield. (00:03:50)
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.
Super Grover ★