As a member of the US Army Liaison Team accredited to the Soviet Army in North Korea, I made a number of trips between Seoul and Pyengyang by train. Usually I would buy North Korean stamps at the main Post Office in Pyengyang, manned by a few unenthusiastic Korean clerks. Sometimes I would put some on cover and have them cancelled. An example, now in another collection, is shown below. I had them with several dates.