That screw does seem like overkill, since the center vent has clips as well. I should have left the screw out.
By the way, be careful with the allen screw that ties the knob in to the vent flap (assuming they are similar between our cars) It is metal screw into a plastic piece: you cannot judge overtightening! I popped thru the back of mine. Snug it until the movement feels okay, or count your turns out, and stay close to that number.
I have what I hope is the replacement part coming, but my tech is worried I have to buy the whole vent. You are forewarned.
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K