Yes, I'd say late 80's to early 90's. Before that there was constant confusion caused by indirect, misapplied or misunderstood terms. Like early 71 or late 71. (how early is early? early in what year? which year - the model year or the calendar year? etc etc. Once we realized that Nissan had documented the 4 model series (1970, 1971, 1972, 1973) in their Technical Service Bulletins we started referring to them.
The first segment of chassis serial numbers sold to the public according to Nissan began at HLS30-00013, the second segment of chassis serial numbers started at HLS30 21001, the third at HLS30 46000 and the fourth at HLS30 120001. It avoided all the misunderstanding related to terms like model, year, model year, calendar year. Series I and Series II were needed to individually identify each of the two 1971 Datsun 240Z offered for sale. Then tell the difference between the two different 1971 Datsun 240Z's in the marketplace.
Looking back at my notes: "The Z Car List" was started in 1987 by Bob Wissman IZCC #176. Before that we had about 300 subscribers on an E-Mail Copy List. The IZCC (a virtual club in the Metaverse) was started by John DeArmond in 1992. #2831 would have been issued in the Feb. of 1996. By 2008 we had over 15,000 subscribers to "The Z Car List". Many new members over the years had just started to look for a 240Z to buy - still asking - what is the difference between early or late 1971 Model Year 240Z's? Which is best? How do I tell them apart? etc etc. (you can see from this discussion that all still goes on today)