Curious Window Hardware on Grove Street

I was in the West Village the other day when I paused in front of this handsome old Federal Style house on Grove Street. Admiring the obvious age of the building's facade, my eye was caught by some metal prongs which protruded from the side of each window. There were two on each side, four to a window in all, and had long since been painted over. What were they? What purpose did they once serve?

Given the age of the building—1820s—I at first thought they were old gas jets which fed outside lamps or lanterns that were no longer there. I have seen prongs of this size and shape that were formerly gas jets. I have a couple in my own apartment.

But then I looked at the building next door, which was in considerably better shape. It had shutters on every window, and the shutters' hinges rested on those prongs. In fact, the first building is the only one of the historic block which has not restored the shutters—and thus the shutter prongs.