For years, it's been one of the Tokyo stock market's biggest open secrets: By the time companies reveal their earnings to the public, the most plugged-in traders have known the numbers for weeks.

Their clairvoyance had nothing to do with superior analytical skills, but stemmed instead from the country's lack of U.S.-style fair disclosure laws.

Listed businesses in Japan can selectively share material information with whomever they want — and it's long been customary for executives to give analysts early peeks into their quarterly results.