I used to braze these back to the frame using a gas torch when they snapped off. They tended to bind up and eventually when worked back and forth they snapped. You need to remove the frame, the remove the glass from the frame, remove the threaded rod, and clean it up before reattaching. Align the rod, fire up the torch. Repaint, reinstall the glass using a rubber mallet and cork/rubber tape. Reassemble the vent to the frame.