The 1857 Capitol extension included room for a Members’ post office. Genteel visitors stroll through the grand space, constructed with masonry vaults and fire-proof cast iron trim. The post office remained in this room until 1870, when the Committee on Ways and Means moved into the space.
History, Art & Archives, U.S. House of Representatives, “Members' Post Office, House of Representatives, Washington, DC,” http://history.house.gov/Collection/Detail (January 04, 2013)
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