As I said to tb727,
121.513 is to only replace the Flight Time Limits, Not the Duty Limits:
121.513 – Flight time limitations: Overseas and international operations: airplanes.
In place of the flight time limitations in §121.503 through 121.511, a certificate holder conducting supplemental operations may elect to comply with the flight time limitations of §121.515 and 121.521 through 121.525 for operations conducted—
(a) Between a place in the 48 contiguous States and the District of Columbia, or Alaska, and any place outside thereof;
(b) Between any two places outside the 48 contiguous States, the District of Columbia, and Alaska; or
(c) Between two places within the State of Alaska or the State of Hawaii.
So a 3 pilot crew under 121.507 has
121.507 – Flight time limitations: Three pilot crews: airplanes.
(a) No certificate holder conducting supplemental operations may schedule a pilot—
(1) For flight deck duty in an airplane that has a crew of three pilots for more than eight hours in any 24 consecutive hours; or
(2) To be aloft in an airplane that has a crew of three pilot for more than 12 hours in any 24 consecutive hours.
(b) No pilot of an airplane that has a crew of three pilots may be on duty for more than 18 hours in any 24 consecutive hours.
But a 3 pilot crew doing international is:
121.521 – Flight time limitations: Crew of two pilots and one additional airman as required.
(a) No certificate holder conducting supplemental operations may schedule an airman to be aloft as a member of the flight crew in an airplane that has a crew of two pilots and at least one additional flight crewmember for more than 12 hours during any 24 consecutive hours.
(b) If an airman has been aloft as a member of a flight crew for 20 or more hours during any 48 consecutive hours or 24 or more hours during any 72 consecutive hours, he must be given at least 18 hours of rest before being assigned to any duty with the certificate holder. In any case, he must be relieved of all duty for at least 24 consecutive hours during any seven consecutive days.
(c) No airman may be aloft as a flight crewmember more than—
(1) 120 hours during any 30 consecutive days; or
(2) 300 hours during any 90 consecutive days.
My understanding is that 121.507(a)(1) & (2) becomes 121.521(a) & (b), but 121.507(b) duty limit 18 in 24 hours was still in effect, Are you saying 121.507(b) is no longer in effect when international?