Visible crew/equipment: When Clara and Opie first sit down at the piano to begin the piano lesson, note there's nothing on the floor behind them. Then that shot cuts to the end of the lesson as the camera pulls back for a wideshot, and now there are two tape marks on the floor near the door in the area where Opie and Aunt Bee will be standing shortly. (00:04:15)
Visible crew/equipment: When Flip pays a visit to the Taylors, he and Andy walk over to Opie, and the actors' two tape marks are visible on the floor between Opie and Flip. (00:21:05)
Visible crew/equipment: After the boys meet Flip Conroy and talk about the football team, when they all disperse and Opie starts walking away the T-mark is visible on the ground.
Answer: Those "boxes" are valences that used to be very common, before air conditioning. They allow for windows to be open during rain storms. They permit air circulation, without letting the rain in.