I’ve been going down a rabbit hole trying to get to the source of the Japanese one. It’s a marker at a shrine with the old name of the shrine, apparently, and no longer has the hexagram. That’s all the further I could get before work, but it shows up in weird conspiracy-like blogs with arbitrarily-drawn hexagrams on maps (using the location of things today despite, y’know, the shrine variously having been purported to move, and some saying the thing was sealed away for over 2000 years which has a whole host of other issues). I’ll report back if I get sufficiently bored. I have a feeling that, short of contacting the actual shrine, I’m not going to get very far. Even the various shrine and tourism websites are mostly dead links that I have to find by searching again on the base tourism site.

edit: This seems like a more sensible site explaining what a hexagram meant previously in Japan (basket weave pattern) https://cultural-experience.blogspot.com/2015/01/blog-post_8.html and addresses the “Japanese people came from the lost tribes” myth that was apparently dreamed up by a Scottsman in the past.