For the purpose of this part, when reference is made to the phrase interest of the United States, it shall mean:
(a) As to vessels constructed or sold with construction-differential subsidy and/or national defense feature allowance under Title V or VII of the Merchant Marine Act, 1936, as amended, the value of the construction-differential subsidy allowance, plus the allowance for national defense features;
(b) As to vessels constructed or sold under Title V or VII of the Merchant Marine Act of 1936, as amended, and adjusted in price pursuant to section 9 of the Merchant Ship Sales Act of 1946, the difference between the pre-war domestic cost and the statutory sales price as defined in the Merchant Ship Sales Act of 1946.