It shall not be unlawful under any antitrust law for any person to perform, enforce, renew, or amend any joint newspaper operating arrangement entered into prior to July 24, 1970, if at the time at which such arrangement was first entered into, regardless of ownership or affiliations, not more than one of the newspaper publications involved in the performance of such arrangement was likely to remain or become a financially sound publication: Provided, That the terms of a renewal or amendment to a joint operating arrangement must be filed with the Department of Justice and that the amendment does not add a newspaper publication or newspaper publications to such arrangement.