Hi! Indeed, according to this : https://itch.io/t/5889447/ito-documentary-research
the early project was developed by Watanabe (with a first prototype, a mock up machine) then Sega decided to not release the project (so would have Stern as it's really niche as you mentionned) Watanabe would have paid to build 3 machines in China .
To my knowledge ; now we have 4 pinballs, one of them is in japan, at least 2 of them are in the USA (including one that may have been rebranded as an X-men pinball machine), and the last one is still not located !