There is nothing subtle about a lot of Communist Vietnamese propaganda produced during the American war. A series of "plane shot down" stamps commemorate, over the course of several sets, the mounting tally of American and allied planes brought down. According to the propagandists, by the time these two stamps were issued on 6 November, 1967 2,500 planes had met their fiery demise.
The hand-stamped cachet/cancel, shown in both red and black, depicts a B-52 bomber with the tail aflame.
I have not previously seen a First Day Cover of this issue.