What Alfred is referring to is the number sequence on the parts books. The small parts books, the ones that came with the cars, are numbered starting with 12, then the sequence of the books. These were intended for the owner to be able to find the part, then using the group and table communicate what was needed to the dealer. The books starting with 10 are the dealer parts books with the full breakdown of the part numbers.
PS - check out the article in the upcoming PWorld for further details. The final proofing is complete and is at the printer. It should be out before the end of the month.