The consent of the Commonwealth of Pennsylvania is hereby granted to the purchase by the government of the United States of such additional lands within the area of Fort Necessity Park, as shall be necessary to carry out the complete restoration of the historic setting: Provided, however, That the consent hereby given is conditioned upon the purchase by the government of the United States of the sites of Jumonville's grave, Braddock's grave, and Washington's Grist Mill, which lands are necessary to carry out the complete historical restoration.