What you are asking about is called "run channel" or "glass channel". Last time I bought some for 55-59 it was flexible corrugated steel, lined with cloth and felt. It lays in a built in door channel with a catch at the bottom. You "train" it around the radius and cut it to butt against the vent window division bar. Bond or screw it in if needed. It's a pretty rough thing to do correctly. Takes patients. You will have to be very careful not to cut it too short. Don't cut it until vent is in and most of the channel is captivated. Press hard into the radius corner, clamp it in the corner, then mark and cut with tin snips. Pull the felt back to cut metal, then trim felt after.
Everything for all of your windows and doors is available. Plenty of how-to's here and on line. Some folks bond it with black 3M weather strip adhesive while window is rolled up. I used tiny screws also. That's real fun.
