That round thingy is splined. The inside of the wiper head, at the top of the wiper, is also splined. It slips onto the head. You have to turn it to the correct position for it to properly be aligned to wipe across the windshield. The arm is spring loaded to force a slight amount of pressure to keep the rubber blade against the windshield glass.
By the way - if you need a refil for a 1937 or 1938 wiper arm, a F@*d Aero mini-van blade will work if slightly trimmed.
Hope this helps you get it together!
Joe
