U.S. Navy special battle efficiency pennant, given to ships which win consecutive annual fleetwide competitions for sustained superior performance in an operational environment.  It is displayed beneath other award flags, if any.  They also paint a gold "E" on the ship superstructure.  For initial (non-consecutive) winners, the regular   battle efficiency pennant (and a white "E") is used.  For more information, see here, here, or U.S. Navy document NTP 13(B) [1], section 1608.
Own work
Carl Lindberg