The Maritime Administrator is prepared to provide an open cargo war risk insurance policy covering any cargoes described in Sec. 308.501. The policy will be in the standard form of War Risk Open Cargo Policy, Form MA-300, prescribed in Sec. 308.517. All policies will be issued by underwriting agents appointed by the Maritime Administrator. All underwriting agents will be domestic insurance companies authorized to do a marine insurance business in a State of the United States.