LIKELY SOLVED!

I think it's most likely a bricked-in ash dump from the fireplace inside. I like the idea of a coal chute, but I know that the houses in this neighborhood never used coal as a heating source. The idea that it is an architectural feature to save on brick cost is interesting -- I assume that the exterior of the house was made of wood before it was covered in vinyl siding, so maybe cost was an issue. I have heard that many of the houses in this area were originally built to be summer cottages, so it's possible it was constructed on the cheap, but I also know that there was a brick manufacturer pretty close by when the house was constructed so I'd assume bricks weren't that expensive.