In 1976, a hand stamp was produced to announce the elections for a "Unified National Assembly" to take place on 25 April. The result of this first assembly was the creation of a single country in July under the banner of the Socialist Republic of Vietnam.
The hand stamp is typically found in red ink on commemorative covers franked with Mien Nam (MN) stamps as shown in the illustration above. The text reads, "Vietnam Independence, United Socialist Ideology, 25-4-1976, Ho Chi Minh City." A variation, from Hanoi, is found on commemorative covers franked with North Vietnamese stamps.
The second cover (below) shows a postally used military cover with the hand stamp applied in black ink, which is unusual. The marking was applied in transit, ostensibly on April 25th (the cover is backstamped a couple of days later).
The red, boxed marking says, "Every voter go vote." This is the first example I have seen with either marking on a commercial military cover.