iPhone 7 - A1660 - Model 3C230LL/A - Why does model number start with a number?

The iPhone 7 I just purchased used has a model number that starts with "3C". There was a comment online that "3A" is for phones that were store demo. And I am familiar with the "M" brand new, "F" reburbished and "N" replacement coding, but I couldn't find any explanation of what the "3C" mean.