The Japanese government plans to sell part of its stake in Japan Post Holdings and its banking and insurance units in a public offering in September, two sources with knowledge of the plan said. Japan Post president Taizo Nishimuro will hold a press briefing on Friday, when the offering will be announced, the sources said. In addition to a network of more than 20,000 post offices, Japan Post runs the country's biggest banking and insurance operations, overshadowing non-state lenders and insurers.