The early vent application can come with a notch and without. Here is a clear view of the early style vent with notch.
http://www.flickr.com/photos/advance-design_parts_co/8129342725/in/set-72157631868452754
And here is the early version without the notch.
http://www.flickr.com/photos/advance-design_parts_co/8129356256/in/set-72157631868452754
I believe this was a running change most likely to address the issue you describe.
Here you can see the significant difference in handles when the interior plates and springs were eliminated.
http://www.flickr.com/photos/advance-design_parts_co/8129367190/in/set-72157631868452754/

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr