GM must have had a good handle on dated mirror supplies and knew how many to order for what...a production week, or month maybe? And if there was an issue at DMI, Ajax's were ordered with the dates for production up to when DMI fixed whatever issue they were having. Is this how it went?
My thought would be that GM just had two suppliers so they didn’t stop production due to being out of required part like a mirror. I am sure the parts were dated coded to track any manufacturing defects like the glass falling out because of bad glue, not that the dated part had to go on a vehicle assembled within a certain time frame. My C AX 9 mirror is on a 5A car.
This supplier issue got me thinking. First, I am sure when GM has a contract with a supplier it is because GM felt the supplier could meet their demand, so something must have happened that prevented the supplier from fulfilling its contract. Could have been a labor issue, fire, or some equipment failure or downstream supplier (supplier to DMI). Ajax may have been another manufacturer supplier, like for Ford or Chrysler and GM may have had contracts that provided a backup plan, IE, different supplier in case of a mentioned unforeseen delay. Or, DMI may have sub-contracted Ajax (more likely), if they could not fulfill their contract. Makes sense to me. I am sure Ajax was not just sitting around with their collective feet on a desk waiting to get back up orders from DMI or GM.
Second, and this is just my experience in manufacturing, is that GM had a standing quantity contract based on engineering calculations of productions in many product lines. I don't think someone got on the phone at, say, Norwood and called DMI to order more mirrors. I am sure it is more a case that once a contract was made, a supplier made X amount of an item per month whether needed or not and more of an automatic shipment schedule that sent out the item on a weekly or monthly schedule.
So, the Ajax mirror, may have been subcontracted through DMI when DMI realized it could not meet its contract demand because of mentioned unforeseen circumstances.