If it is like my '40, metal frame on outside has screws on inside of metal cab liner. Remove screws and metal piece - may take a little prying so be gentle. That leaves rubber gasket which I ended up cutting out with an Xacto knife ( rubber was so old and PO had glued on top to stop leaks )


1940 Chevy KC 1/2-Ton
In the Stovebolt Gallery