It looks like it might be the same as AD defroster vents, with a clip on one end and a single screw on the other.
Brad Allen has a pic of a 41-48 Defroster vent that has the same thing HERE.
You might scroll thru more of his pics. It's a treasure trove of stuff. thumbs_up

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.