How about this theory....There are 12 numbers in the UPC bar code on a Lionel box. The 6 means the first 6 digits which is the manufacturer which is always the same. The next 5 are the 5 digits of the product number and the 12th digit could represent some unknown item like production run or if the item has a different number car numbers???
What makes the new system more complex is that the car or engine number is not printed on the side of the car like in pre and postwar so moving to a UPC based system makes sense,