1957 AC Dash Vent

[Belair-o]

I find that the diffuser grill that rotates on one of the aftermarket AC vents for my 57 Chevy is way too tight. Anyone have any thoughts on how I might free it up so it rotates easier? I installed it some time ago, and have been hitting it with penetrating oil, but that hasn't helped. I am not sure if the vent can be disassembled and clearances opened up some? Perhaps someone has experience with the vents?
Thanks, Doug