Among the Kodachrome slides that belong to my mother in law are several from a trip to Washington, DC. Guessing from a number of clues among the entire set of slides I scanned, I think they’re from about 1948. Certainly no earlier than 1947, and no later than about 1953.
Three photos probably taken from the observation deck of the Washington Monument show a very different National Mall than we experience today. The Lincoln Memorial and its reflecting pool, and the US Capitol and the grassy areas before it, were there. But so were a number of buildings not present today. Check it out:

The buildings on the left are a grassy area today. The buildings on the right have given way to Constitution Gardens and its pond. These buildings remind me of other buildings I’ve seen only in photographs that were built hastily as office space in support of World War II.
The Vietnam War obviously hadn’t happened yet, but it would happen, and eventually the Vietnam Veterans Memorial would be built beyond the buildings on the right. Finally, the National World War II Memorial would be built some 55 years hence, replacing the small pool before the Lincoln Memorial Reflecting Pool.

Looking east toward the Capitol, you can see that most of the Smithsonian museums haven’t been built yet. More of those anonymous-looking buildings stand beyond the Smithsonian Castle at center right. That’s where the Hirshhorn Museum, the National Air and Space Museum, and the National Museum of the American Indian would eventually go.

One last Kodachrome from atop the Washington Monument shows Virginia Avenue and the Potomac River. The set of buildings in the bottom right corner is the Department of the Interior.
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