One screw in the side of the center vents (as you said), one bolt in the center vent and a lot of clamps (2 in the center vents and 4 at in each side vent).
In the center vent is a bolt connected to the valve opening and closing the air-ducts. If you look closely over the handle you can see it; untighten this one as well.
If you look in the vents, you can see the clamps; these need to be pulled up (or down concerning the clamps in top of the vents) simultaneously and the vent can be pulled out.
greetingz & good luck,
with a minor 600+k