These plaques are affixed to many buildings in Old Town. The Historic Alexandria Foundation is a nonprofit organization founded in 1954 to preserve and restore historic buildings in Alexandria. Beginning in the 1960s, they awarded plaques to homes at least 100 years old which have maintained their architectural integrity and detail. In later years, homes had to pass an inspection to be eligible for a plaque. In the early years, the plaques were made of aluminum. Later, they were bronze.
If you look back at the house with the powder blue gate, you will see one of these plaques on the wall below the light fixture.