IsZo, a New York-based firm with roughly $100 million in assets, is taking on Nam Tai, a nearly $300 million company which invests in and develops properties in China. The company counts index fund heavyweights BlackRock, State Street and Vanguard and the California Public Employees Retirement System, the largest U.S. pension fund, as investors.

The activist, which owns 9.8% of Nam Tai, wrote to others Monday to try and rally a group who own 30% of the outstanding shares to join a call for a special meeting.

In the letter, seen by Reuters, IsZo's founding partner, Brian Sheehy, accused the board of conflicts of interest, backing a questionable acquisition strategy and having no credible plan to boost Nam Tai's share price, which has tumbled 25% in the last 12 months.

A representative for Nam Tai did not immediately respond to a request for comment.

The letter said if investors succeeded in getting a meeting they could then "remove five of the incumbent directors ... and appoint IsZo's six highly qualified and independent director candidates."

IsZo's directors would bring real estate expertise, experience operating in China and capital allocation and transaction acumen to the board, the letter said.

Sheehy said Nam Tai's biggest investor, Kaisa Group Holdings, has helped stack the board and management suite with insiders. Nam Tai's CEO, Ying Chi Kwok, is the younger brother of Kaisa's chief executive.

"We believe reconstituting the Board and removing Kaisa insiders from leadership roles at Nam Tai is the only way to end this dangerous and destructive tailspin," the letter said.

(Reporting by Svea Herbst-Bayliss; Editing by Cynthia Osterman)

By Svea Herbst-Bayliss