This seemed very unclear to me at first, until I tried to replace the cart tray. I apparently didn’t get the little lip under the mobo on my first attempt, and I noticed there was a pretty decent chunk of space between the front brass screw holes and their corresponding holes on the mobo. I reseated it, and the second time I heard a little click (apparently the lip going under the mobo). This time the bottom/brass screw holes were virtually FLUSH with the mobo holes. If you see a chunk of space between the cart tray and the mobo near the brass screw holes you didn’t get the lip under the mobo.