The original, 1917 version of the U.S. Navy Merchant Marine Naval Reserve Pennant, which per 10 U.S.C. § 7225 can be flown by vessels which have been designated by the Secretary of the Navy as suitable as a naval auxiliary vessel in time of war, and the commanding officer and at least half the officers are members of the U.S. Navy.

This is the version from when the flag was first defined by U.S. Navy General Order No. 285, on April 14, 1917.  Today's version of the pennant is basically the same, except the eagle's head is turned the other way.  For more information, see SeaFlags, and Navy Document NTP-13(B), section 1717.
Self-made, following the specification in File:USNavyMerchantMarineNavalReservePennantSpecs.png, with the eagle being traced from that specification and cleaned up.
Carl Lindberg