Where in Washington?

Who knows what this is:

Do you recognize the handsome structure pictured on this postcard from the collection of Jerry McCoy? If so, send its name to me at kellyj@washpost.com. The person who sends me the first correct entry will win a Post Pulitzer postcard.

Last week's winner was Ali Bourque, who knew that the postcard showed the old Patent Office, now home to the Smithsonian's Portrait Gallery and Museum of American Art.