An example is the depicted cover that bears a nice "Hai Quan" hand stamp and is postmarked Hanoi, 1958. Thinking this might potentially be a North Vietnamese naval military cover, I put in a bid. If nothing else, the fact that it was sent to China was interesting.
As it turns out, there are no accent marks on the term Hai Quan in the hand stamp, so it translates as "customs organization" as opposed to Hải Quân, meaning "navy"
The sender (and recipient) however, held a surprise for me. Rewi Alley, a native of New Zealand, was a writer, educator and member of the Chinese Communist Party. It isn't clear what he was doing in Vietnam in 1958, but it could have been related to his works published in 1956, Spring in Vietnam, a Diary of a Journey and Buffalo Boys of Viet-Nam. Alley is credited with coining the term "gung ho".
So, it turned out to a be a worthwhile cover after all (the stamp isn't bad either...)