The pictures shows an aluminum trim piece, I was told that that was the original trim used to hold down the canvas. There were snaps at the bottom to seal the back when in the closed position. My 1948 that I sold had snaps (actually little posts that swiveled to lock) and did not have the trim. I bought it from the original owner who had them put on when he put on the new canvas for an easier removal.
I have a colored postcard that shows a 1951 GMC with the black canvas rolled up. I cannot see the fasteners.
I know that the Canopy Express in the photos was original, I just sold that one and I kept the trim for my 1949 Canopy Express. When I talked with Neal49 about that, he confirmed that the aluminum trim was original from his experience. With the trim installed, there was no way snaps could be used without drilling holes in the trim.
But as you know,there is lots of information out there and some of it is right. This is to my best recollection of information that I have gathered over the years and should be researched to prove it's authenticity.
picture showing same trim Picture showing green canvas Picture showing Black canvas