@Mymando I'm no expert and correct me if I'm wrong. The chance of Schwinn making 2 identical frames in '58 one heavy and the other middle is slim. Schwinn did make a fat bar straight bar early on and was for sure a heavy weight. The major difference of middle and heavyweight is the wheels. You can run S-2 wheels and tires on even the later middlweights as I earlier stated. The fenders on heavy's are wider, but not always the frame as well. The fork spread from heavy to middle is different and also the seat /chain stays but not always. I believe in '58 it was the same frame just different accessories.
@GTs58 am I completely wrong about this were there 2 different straight bar frames in '58?