If the Secretary of Transportation finds that the operation of any passenger vessel with respect to which a contract for the payment of an operating-differential subsidy has been entered into under section
1173 of this Appendix effective before January 2, 1960, is not required for all of each year, in order to furnish adequate service on the service, route, or line covered by such contract, he may amend such contract to agree to pay an operating-differential subsidy for operation of the vessel