CO mystery - any ideas?

Bruce Cunningham wrote:

the cure is sometimes that in the rear
inspection covers on the sides of the tail in front of the stabilator,
they have a edge bent outward to scoop air into the tail and keep
exhaust out. These scoops or inspection plates have to be mounted with
the scoop facing forward so that they face the airstream. People have
installed the covers with the scoop facing down or backwards, usually
after an annual etc., and then they start getting CO in the cabin.

Are these the round inspection plates like those one finds all over the
airplane, or something different?

Is the "scoop" really noticable, or a slight bending?

- Andrew