Never saw those markings before, either. For whatever it's worth, the present day listing for the street address in Washington, DC is the Newman Catholic Student Center at George Washington University.
I have no way to necessarily connect it, but the addressee "Mr. Philip R. Hough" is the namesake of the Superintendent of George Washington's Birthplace/Museum in the 1930's. It could possibly be the same person, given that your scanned cover is from around the same time period.
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