1938 Cadillac 60-special. The first modern sedan.
Notice, there is a clear belt-line. Also, the transition from the C-pillar to trunk is a true notch-back shape. Also, this is one of the first examples of pure chrome window surrounds. All in all, the first real 3-box shape. Front (hood), middle (body), and rear (trunk).
Really. Find one before this.