I guess I must be particularly blessed! I have an old time locksmith shop in the next town that can fit keys to just about anything I bring in to them. I have brought Gerstner (Eagle), Yale, Rockford, and others to them and they hand fit on key and then duplicate the second fro it. Besides, you can’t really depend on lock codes to be consistent with the amount of wear on the lock.
This guy has the 56E4 you are looking for”
http://islandtrunkshop.com/antique_vintage_flat_keys.html
Scroll down to National Lock.
Terry