Chrysler was famous for implementing upgrades during a model year, especially in the 50s-70. The difference with Chrysler compared to the other companies was those improvements were usually implemented as a unit and were changed out on the production line. A part number change often designated that a unit was upgraded, or they changed a supplier, but usually the old part number worked in place of the new part number, but not always. Chrysler also did a lot of part number reassignments, part numbers were superseded to a different number, sometimes simply because of a model year change, sometimes several times, but the part itself may never had changed.

This was especially a problem with larger trucks, some may have set on dealership lots for a long time and may have missed a few running changes that may have happened on the production line between the time the truck was built and the time it was sold. The bigger the truck, the more the issue became apparent. Then add the ability of the customer to pretty much order a vehicle to be built any way they wanted it, some truly unique parts got installed on some vehicles.

As a general note, GM usually held back upgrades until the new model year, very few changes were made during the model year, until recently. Ford would make changes on the production line as well, but usually at Ford, the changes were usually something was completely different, and nothing from the old transferred to the new. Running changes at Ford were based on a date, or a serial number of the vehicle. Sometimes Ford would change a part 2 or 3 times in a model year, and then at the model year as well, but nearly always, a part number change meant the entire deal wouldn't work with the corresponding part numbers that changed with the one. If you needed Ford parts, you needed the build date, the serial number, and a part number, to get the correct replacement, then hoped they still made the part you needed.