Weiphone Forum (China) has posted a picture of the box barrel machine called "iPhone 5C" as shown below. There are no sources to verify this is the product from Apple's production lines or just a fake from China.
If this is the box of iPhone oF Apple, the letter C will represent the color (Color), resin (plastic), polycarbonate or cheap (Cheap.) This interpretation is entirely plausible that Apple used the letter S in the product name to denote the new version has higher speed and other upgrades.
According to the image above, the box will not use paper products, such as the previous iPhone that will use the same plastic shell iPod Touch. This is the material that will appear on the cheap version of the Apple iPhone.