In this movie, the top side edges are straight.
In the actual TV show, the top side edges are curved, like every other live-action incarnation of the Superman costume.
"Okay, first of all, it wasn't 'Eek, a mouse,' it was 'EEK, AN OIL TRUCK!!!'"
Actually, in the later colour seasons of the show the, S became more straight. They just didn't bother using the rounder one for the black and white costume.
http://a1.files.biography.com/image/upload/c_fit,dpr_1.0,q_80,w_620/MTE5NDg0MDU1MjA1MDg2NzM1.jpg
http://www.authortonypiazza.com/wp-content/uploads/2012/11/sman1.jpg
http://www.gettyimages.ca/detail/news-photo/the-superman-costume-george-reeves-wore-in-the-50s-tv-show-news-photo/2348542?Language=en-GB