All construction in respect of which a construction-differential subsidy is allowed under this subchapter shall be performed in a shipyard of the United States as the result of competitive bidding, after due advertisement, with the right reserved in the Secretary of Transportation to disapprove, any or all bids. In all such construction the shipbuilder, subcontractors, materialmen, or suppliers shall use, so far as practicable, only articles, materials, and supplies of the growth, production, or manufacture of the United States as defined in paragraph K of section
1401 of title
19; Provided, however, That with respect to other than major components of the hull, superstructure, and any material used in the construction thereof,