Howdy,
The original 56-56 civil war guns were also called .52 caliber, as that was the diameter of the bore inside the lands. They usually slug out at .535 to .54 in the grooves (the two I have owned).
The 56-56 was originally called the number 56, based on the case diameter of .56, I believe. It was not until after the .56-50 came out that the started refering to theee Civil War ones as 56-56 if I remember right from Marcot's book.