They finish the top and bottom, this piece was phased out by mid-49, I like them so I added some to my '50. The early style surrounds carry a small tab they screw into that the later versions do not, they went to just a flat tab like these pictured.
https://www.flickr.com/photos/advance-design_parts_co/8129177743/in/set-72157631867396207
https://www.flickr.com/photos/advance-design_parts_co/11590024186/in/set-72157631868452754