In the 40's and 50's (even seen one pre war bike) the rear brake cable would from time to time be routed through the top tube, placements of the racetrack shaped reinforcements at the entry and exit used would be often on the side from 30 degrees above or below the horizon line of the tube. That is what I think may be visible near the head tube along the top tube. A hunch. This is where One image only is of no help at all.