It is Japanese, but it's not ancient, it's not a coin, and I'm pretty sure it's not genuine.
This example on zeno.ru looks like yours, and the discussion about
this "tree" of them explains it: they are "fantasy tokens", no older than the 1920s. These examples on zeno.ru are 50 mon ("wen" is the Chinese pronunciation of the same character), yours is larger and if your informant is correct, a fantasy 250 mon token.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis