In general, a service part number is an inventory number. It represents a part on a dealer's shelf; it has an associated price; it has a minimum and maximum stocking quantity; it should have a where used list; it is a cross-reference part number; it contains a source part number for re-supply. It's that last part that gets you what you need, not necessarily what you want, Ford maintained service part to keep cars running, not concours correct. The service part number, if it was propperly used, should list all the engineering part numbers it replaces. To further complicate the process, waivers may have been granted by Ford as to part marking (a label vs an indented stamp), tolerances, color, materials, manufacturing procedures (read government mandated "improvements"), packaging, substitutions, etc.
As to the part number itself, I cannot think of any instance where a part is permantly (not inked stamped) marked with a "Z" in the fourth position, (C5ZZ-xxxx-x), and is considered NOS, service or a factory part (or assembly). If you do find a permantly marked part with a "Z" in the fourth position, avoid it if possible. The part is a (poor) reproduction.
Jim