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When a part is first designed, engineering assigns it an alphanumeric part number, such as C5SZ-1005-A.
If the part is later completely redesigned, a new part number is assigned to it by engineering, to differentiate between the two parts, for instance C8AZ-1007-E.
This could mean revisions to the part or multiple manufacturers supplying the part to Ford. Note that this switch was originally designed for the Mercury Division, due to the "M" as the third digit in the part number.
It is also a unique number, and changes due to specific equipment options on a given vehicle.
According to the parts book, this switch fits a 1968 Ford Thunderbird with tilt steering wheel.
It is actually quite common to find parts originally designed for Thunderbirds on Lincolns, and Falcon parts on Mustangs.
In order to spread out the costs of designing and manufacturing a part, Ford used parts on different models.