Update: I looked at this block more extensively in early March of 2020.
Nowadays many historic neighborhoods have strict building codes that restrict architectural styles. Builders now find themselves building in styles that were popular one hundred years ago, and many do an excellent job of reproducing historic styles. I was almost fooled by these row houses on the south side of Lafayette Square, even with the near perfect brickwork; after all, maybe they had just done a really great job tuck pointing the old brick.
But as I look on the side of the house, the truth became obvious-the house had a poured concrete foundation. Definitely not from the late Nineteenth Century.