The U.S.S. Roper was a destroyer built in 1918 and used in World War II before being sold and scrapped in 1946. This card was mailed in July of 1942, and the sender noted that he should have sent this card three months earlier. At that time in April of 1942 U.S.S. Roper destroyed a German submarine, and a confidential report filed the day after the event can be read at U-Boat Archive here.