Well, there definitely was a Sante equipped Altron 7000 cataloged in 1988. It's ironic that you mistakenly posted the serial number using what would have been the correct year identifier!

It not abnormal to see a year code from the previous year but two years prior is atypical. There are several possible reasons. While I wouldn't rule out your scenario, I think it's more likely that the forecast volume of sales did not materialize and some frames were carried over and finished in the following year's livery. Another possibility is the frame got rejected for some reason and got caught up in a rework cycle. Finally, somebody could just have used the wrong stamp.

The serial number decryption is a dymnamic process. They'll always be exceptions. Some will be eventually explained and some will remain mysteries. Even in cases of brands where we have a factory supplied key, anomalies still pop up. I certainly would like to know both K and W prefixes were being used on Kawamura built product for the USA market after 1984. Regardless, the system seems to work in the vast majority of cases.

BTW, the 1988 Altron 7000 I've seen are more of a 3 colour fade; royal blue to medium blue to white.