The Secretary of Agriculture is authorized, subject to such conditions as he may prescribe, to transfer, without reimbursement or at such prices and upon such terms as he may impose, to States and political subdivisions or agencies thereof fire lookout towers and other structures or improvements used by the Forest Service for fire prevention or suppression purposes, and the land used in connection therewith if such land is outside national forest boundaries, when they are no longer needed by the Forest Service for such purposes but are of value to the State or political subdivision or agency thereof in its fire protection system: , That if any property so transferred is not put to use for the purpose for which it was transferred within two years from the date of transfer, or if, within fifteen years from the date of transfer, any such property should cease to be used for the purpose for which it was transferred for a period of two years, title thereto shall revert to and immediately revest in the United States.Provided