Subchasers in column of divisions, SC 291 on the left. See also this photo, probably from the same convoy.
Caption: "SC Chasers in Column of Divisions. 291 to the left. Also being left, I had bum luck this day as one of the motors was out of commission."
From set of photos purchased in Chicago in 1959 by a resident of Minnesota. Thanks to K.S. Christian for permission to post the scan.