“Not of The Line Vice Admiral Flag” and was likely used aboard BM3 Hile’s boat, and not the Calvert itself.
Further explanation regarding Not of the Line for us non-Navy folks: “Those not eligible for command at sea include officers of the staff corps (medical, supply, chaplain, civil engineer, judge advocate general, dental, medical service, and nurse corps) as well as line officers restricted to engineering or special duties are not of the line officers. These flags are never flown aboard ship, but may be flown in boats and are used ashore to designate the headquarters of such entities.”