Unfortunately it looks like piano sellers are as much inclined to make their pianos younger as guitar sellers are to date them back. However, my preliminary impressions from the small sample size of uprights and baby grands is that piano production ended, let's say in the late 80s. There are different serial number schemes for the upright and grand pianos and it's not entirely clear if the production of the baby grands ended earlier than the uprights.
Here's the baby grand 5-digit serial scheme, Ynnnn is pretty plausible with the 2nd hand sales offers. I think 1985 is the latest I've seen so far.
This most commonly seen 6-digit scheme for the uprights is more confusing though:
If that's the usual scheme, it could mean this example was built in 1985. However, the seller of this one claims "1984", another 5nnnn piano was sold as "1986". And then again there's pianos like this:
Piano production started 1978, but what looks like a high production number that would indicate it's rather 1988. But of course there's also a 5-digit scheme to find on Tokai uprights:
That may or may not indicate 1980, and maybe the "1988" above means actually 1983, maybe this is all wrong, however the sellers of the handful of instruments I could currently find all state a build year in the 1980s.
Why is this important? The end of the piano production is probably the best indicator for when the original Tokai factory in Terawaki-cho ceased production. Everything guitar-related points to the late 80s, and the end of the piano production may help narrowing that down a little more.
Reading through the piano forums brought up surprisingly controversial assessments on how good they are, but also the interesting hint that they made an electric piano in 1975, which was basically a year before Yamaha rolled out the CP70/80 series.