Opie Flunks Arithmetic - S5-E30
Other mistake: When Aunt Bee walks out of the grocery/meat store the door's located between the grocery's one display window and the TV display window, but the grocery store actually has two display windows with its door between the two. This was shot on the soundstage, not backlot.
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.