The numbering is quite logical since the 1600 is a mid range board, successor to the 1500, which was a successor to the 901. The Panasonic KC were entry level "Christmas Present" type affordable keyboards to get kids interested in music. It still has some followers today but mainly gave us the trackball on the 6000/6500. The WSA1 was an attempt to break into the synthesiser market with an interesting acoustic modelling approach.
The AC1200 was just a KN1200 in a box without keyboard or speakers and was quite popular. The AX were amongst the first affordable PCM sampled keyboards, at the time when Yamaha were still using low operator FM synthesis as their sound generators, and the sounds were a huge step up in quality in relative terms at that time. Hats off to the people who worked in the MusicalInstrument Division of Technics/Panasonic. What a story of advancement in a short number of years. I would appreciate it if someone could respond with the actual historical lineage of the keyboards from the KN600 - 800 up to the KN7000. It won't wear out ( my others did't) and it will challenge my playing and creative juices for as long as I live. It will probably take the rest of my life to exhaust my use and uses for this fantastic sounding instrument. Then the KN7000 and I waited until last April and could not resist the desire to have that incredible giant step forward in all that it had to offer and especially the SD card. As newer models became available, I kept thinking that every two years or so there would be a newer / improved model.
Then I bought the KN2000 (still have and use) and I thought that was everything I could want. My first Technics keyboard was a KN800 and it had it's share of quirks, but my next Technics was a KN1000 (stil have and use it), wasa giant step forward and great improvement.