The truth is rarely pure and never simple.
— Oscar Wilde, The Importance of Being Earnest, 1895
The image is of a truth window! I had never heard of the thing. I have seen simpler versions, often in renovated heritage homes, though I never realised they had a specific name :)
“A truth window is an opening in a wall surface, created to reveal the layers or components within the wall. In a straw-bale house, a truth window is often used to show the walls are actually made from straw bales. A small section of a wall is left unplastered on the interior, and a frame is used to create a window which shows only straw, which makes up the inside of the wall.”