I cannot find a way to remove the ballast cover on one of my fluorescent light fixtures. The cover is in the middle of the fixture, and there are wires coming out of the endcaps going to the lightbulbs. (Unlike other fixtures, I do not need to remove the light bulbs to access the ballast cover.)

There looks like there is a screw with no head that the cover needs to slide off, and there is a small black flat piece that LOOKS like I should be able to do something with. There is no nut or clips on the side (in fact the ballast cover doesn't even top the top of the fixture.)

Does anyone know how to remove this?