What about adding an extra field for cartridge/disc number? A lot of games have a separate box number and cart/disc number. For example, a game released in the UK and other parts of Europe may have the same cartridge number, but a different box number due to a translated cover and/or manuals. Or a US game that was released both as a regular and limited edition, where the cartridge is the same and the only differences are added merchandise and a different outer box.
I believe the current preferred method is putting the box number in the Item Number field and the cart/disc number in the Description, correct? But I have seen a lot of cases where people don't seem to understand this and put the cartridge number in the Item Number field, for example. It does not appear very intuitive to me at least. It also leads to incomplete entries with only box numbers where people just leave out the cartridge number because it is not being asked for, even though cart numbers are crucial information to some collectors.
So in short, I personally think it might be a good idea to introduce an extra field for the cartridge/disc number (and along with that maybe rename Item Number into Box Number or something similar). Let me know what you think!