Re a Part 129 air carrier that is short of co-pilots: Can a pilot who usually flies in the left seat (B737 aircraft: non-US registry) as the Pilot-in-Command fly in the right seat as the SIC (co-pilot)?
From the regulations:
Each person acting as a flightcrew member must hold a certificate or license that shows the person's ability to perform duties in connection with the operation of the aircraft. The certificate or license must have been issued or rendered valid by:
(a) The State in which the aircraft is registered; or
(b) The State of the Operator, provided that the State of the Operator and the State of Registry have entered into an agreement under Article 83bis of the Convention on International Civil Aviation that covers the aircraft.
Since a captain can always fulfill the duties of a first officer, then yes, someone who normally occupies the left seat can occupy the right seat.