The cover for the 590/595 is about 2x3 inches and fits the mount perfectly dimension wise. Top and bottom have a small curvature toward the outside edges. The release is activated at the top right of mount and controls/releases four pins at the top of mount that hold both the cover and the unit in place. The bottom is wedged in place. It is quick release. I do not know about the 395 but it is very different from the 550 I use to have.