The SCHED and ITEM NO slots were used to denote when, in a particular batch of cars, that particular one was built. A schedule was put together to build a particular type of car...2 door...4 door...blue...green...whatever. Then, once the schedule was ready, the number of cars to be built was determined...that's the item number. I forget the actual allotment, but for each schedule, a certain number of cars were constructed. Each schedule might take more than one day to complete, but generally, the item numbers were in the few hundred range. So, the lower the item number, the earlier in the schedule it was built.