Per Bob Manell's Small Block V8 book, the RH manifold casting number is C6OE-9430-F and the LH manifold is C6OE-9431-F. Now it gets interesting. Service part number are D4DZ-9430-E, RH, and D0OZ-9431-D per the 75 Ford Car Parts book. Those would be marked on the box, not the part. Later years may have a different part number. There's no telling what is on the actual manifold. Your manifolds P/Ns were used from 1966 to 1969 on all Ford and Mercury cars with a 302 (a 68 "C" code too) so if you want exact, a date code earlier than 6D16 would be your goal.
Jim