Well, as far as I know, some Zs, like the Fairlady Zs, didn't have a build date stamped anywhere on the car. I don't know if that's also true with european models as well. If there's no plate on the door jamb with a build date, it probably isn't on there. You might try checking your seatbelts if they're original. They should have a date on them somewhere. I know there's other markings that could tell you, but I think the seatbelts might be the easiest.
However, the date on the seatbelt is not necessarily the build date of your car, but it should be close, if not the same.