Someone (sorry, I'm quick with the delete key) was discussing the 
mechanisms holding the vent rod to the flap and described a slotted set
screw with a nut for maintaining the "tightness".  I have to replace
the slotted screws on mine since one side of the slot was broken and the 
securing nut was rusted on and reasonably unmovable.  (As one might expect
given a leaking air vent flap.)  I'm replacing the slotted screws with
allen-head ones becuz (1) its easier to get an allen wrench within the 
confines of the duct-werks and (2) becuz I can easily get these types of
screws.  I replace the screws holding the cover plate with allen head 
screws also.  These look really neat.

christian