Early/late is too vague. Without context, it could mean anything. To simply say you have an 'early' Z could mean anything. It could mean any 240z, any series one 240z, any 1970 series one car built early in the year, or any series one car with a 1969 build date. In the context of 1969 build date cars, it could even mean only very early build date 1969 production cars. When you refer to a Z as a series one, most people who have been around here for a minute know exactly what you're talking about.
And I would argue that there is a clear cut off. For US cars, VINs under, I believe, about 20500, build dates up to 1/71 - although recently there was a late production series one car that had a 2/71 build date, but that is an anomaly, and I've never seen such a thing.
It works just fine as a 'series' descriptor, has for many years. I certainly wouldn't consider myself an authority on the matter, but I think my definition of a series one car is pretty accurate. It is not meant to apply to cars with an exact selection of parts. Also, I would only apply it to US models. I don't know the non-US Z market that well, so maybe it could apply to cars outside of the US market, but I am assuming that the term originated in the US, and is best applied to US market cars. Someone else can chime in if they feel differently.