Also, as mentioned above, there are some 1980 horns that have serial numbers in the form of "xx80xxx." That's an interesting kicker!
Looking at my timeline, you can see how easy it is to get confused. As a good example, the S-880 was (almost) definitely released in 1985. People then conclude that all 880s were released in 1985. Yanagisawa doesn't like releasing all their pitches at once. Hey, most manufacturers used to release baritone models well after the new model got established.
Another reader sent me an Excel spreadsheet of a couple hundred sopranos. I haven't really looked at it yet -- still working on the Gallery -- which should give me a little more info on the soprano lines and some more serial number data, too.