I have a 1971, and mine was screwed on. It worked fine but I don't know if it was original. I took it off cuz I have to deal with the drip rail rust now. My G10 is the only American vehicle I own. When I got it I thought parts would be easy to get for an old Chevy. How wrong I was, so hard to find and confirm what parts are needed, especially when it's hard to tell if the part I'm replacing is the original one.
I have a 1976 G30. It is not a shorty but have some pictures of mine. My cover has 2 holes that screw into the hinge block and a lip on the front that slides under a tab secured by a screw. Pictures are better than words in this case.
I did not look at how it is attached, but I took a picture of the cover on my 79 yesterday and sent it to Kursed to post here... It has the sharp lines instead of being radiused...My van also was ordered originally with the extended slider (11").