The U.S. Navy Infantry Battalion flag, which for many decades served as the de facto flag of the U.S. Navy as a whole, until an official flag was defined in 1959.  Today, it is used as an organizational color for a ship's landing battalion and similar units during ceremonies.  The flag dates from sometime during the mid to late 1800s, but was not defined in Navy signal books until 1900 or so.   There was once a red version used for artillery battalions, but it was discontinued after World War II.  The size of the flag is 61 5/8 inches by 78 inches. Units may add their name and location in white sans-serif letters above and below the anchor.  For more information, see here, here, and U.S. Navy document NTP 13(B) [1], section 1711.
Own work (anchor design traced and cleaned up from File:USNavyBattalionFlagSpecs.png)
John A. Dahlgren