Oh you're really asking for it!
Listen, this only got started because humans like to have names for things, and we are intrinsically lazy. The big exterior obvious differences with cars that do or do not have the hatch vents, pillar vents, vertical def. glass, original console, and the big perceved value, rarity and coolness jump between "early" and "late" 240's that have these big obvious differences, required our stupid human minds to create a simple way of expressing ourselves when we wanted to say "the cars with the hatch vents, vertical lines, original console" but since we're lazy decided on "Series 1". I'm surprised no one has claimed to be the first to use the term. Tweren't me.
Then things got crazy when there were so many other changes, some small, some big, that happened as the cars evolved and individual country laws evolved that required the cars to evolve in unique ways across the globe, as well as Nissan deciding to add value with more features and power. As we've discussed there are likely 7000 small and large incremental changes most of which do NOT constitute any reason to give something a new name.
I'm happy if I only ever hear these terms to give us clear and simple names that we can use for discussion. S1 240, S2 240, 260, and 280. Hell, just use the year. I have a 70. I have a 71. then if anyone cares, tell them if you have vert def glass or a late console. It is impractical to arbitrarily decide what specific changes constitute adding a another number to the series series.
Frankly the more I think about it, its really all about making everyone think your car is better than theirs just cause it has all that early stuff or is special. That's why I'm busy sharing all the cool unique stuff about 3798 and 2744 cause they're Canadian and have unique stuff AND they're early, AND I found two of them! Na Na Na booboo. I don't spend hardly any time telling you about all the stuff on my 73. (Unless I'm trying to sell it, then look out!) Damn it, humans are crazy beings.