Here is another cover I picked up at the NAPEX show. I couldn't resist the lovely franking, despite the cover's awkward size - it took two passes through the scanner to capture it and I'm not sure how to store it (click the image to make it larger).
It looks like there may be a stamp missing near the bottom based on the partial postmark, so it's probably not possible to accurately identify the rate paid. It presumably contained documents or photos that were not to be folded.
The return address is the American Embassy in Laos. The cover was sent in May 1958 by a member of the US Observation Mission (USOM).
USOM Laos was established in January 1955 to provide foreign aid.