First of all, there really isn't a '69. It's common for cars of a given model year to be built towards the end of the previous year. Therefore the early 1970 cars were built in late 1969. Since the earliest Z cars are considered more desirable those built in 1969 are certainly the earliest.
In the USA, at least, 1973 cars had to meet more stringent emissions regulations. Nissan used so-called "flat top" carbs for '73. Many people don't like thes and instead prefer the earlier "round top" carbs. Also, the bumpers for 1973 were pushed out from the body a bit to improve crash worthiness.
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