Woodland Hills-based insurer 21st Insurance Group received an offer Wednesday from American International Group Inc. offering to buy 38.1 percent of the company for $690 million. The price works out to about $19.75 per share in cash, a 19 percent premium to Wednesday’s closing price of $16.59 a share. AIG already owns 61.9 percent of the company. The purchase would make 21st Century a wholly own subsidiary of AIG. The company made the offer in a letter to 21st Century’s board. AIG has said does not want to sell or dispose of 21st Century. “Our proposal represents an excellent opportunity for 21st Century’s shareholders to monetize their investment at a full and fair value for their shares. For AIG, this is an opportunity to make a substantial additional investment in a business we know well,” said Martin J. Sullivan, AIG President and CEO in a statement.