What is JP-21 (Japanese Scart)
Looks like it is a standard although I couldn't find a copy of it, it's called TTC-003 of the EIAJ standards of Japan.   Looks like EIAJ merged to form JEITA

Here's a link that seems to have the pinout.   It looks like TTC-003 was part of a larger standard, maybe someone will know more.  Keep in mind though it looks like this was around in the 80s so it's pretty old now and pre-dated the internet so maybe there isn't a digital copy.  Could be worth asking Jeita if you really want to know.

This is just a guess but it may have been superseded by this D-terminal connector.