EBay doesn't seem to use those codes much anymore (if something that precise ever had a specific code).
I do remember having specific codes for various types of books: Humour, Militaria, Science Fiction. The codes even distinguished between SF and Fantasy.
Now, all of those are lumped as "Fiction".
Do you mean the UPC or MSP?
If that is not on the Advanced Sell Your Item form where you can copy it from the packaging or manufacturer's website, you can add your own Item Specific to give the information.
Item Specifics are a secondary level of Search and show up on the left side of the List manufactured by a Search. That is populated from Item Specifics.
If you have been defaulted into the horrible overSimplified Sell Your Item form, look around at the top right for any blue links to find the Advanced.
It is soooooo much more useful and has more options.