Spot the change? The old school window panes have been outlined with black rectangles :) I thought it resembled the Chinese character "目", which means sight. The window panes do allow you to gain sight literally of what's outside by manoeuvring the lever on the inside. Hence, the play of the pictorial word "目" with accompanying words "peeping" through the openings of the window.
I took this picture at the National Museum of Singapore. It is an exhibit displaying a mock up of the exterior of our early HDB houses.