North Vietnam, Eighty Years Day of Labor, May 1st, 1966, Michel Nr. 443; A single stamp in a 6 xu nominal and perforation of 11.0 was issued. The 6 xu tariff was likely used to cover the standard post card rate within North Vietnam and to fellow socialist countries.
Rare multiple franking of the 6xu value paying the correct 12xu postage on a letter sent to Czechoslovakia mailed in August of 1966.
Here is a letter that features one of the stamps and that shows an interesting mixed currency franking of old and new Dong (ex Klewitz). The railroad stamp on the reverse of 100 Old Dong was worth only 10 xu in 1966 (after having devalued 1,000:1 in March of 1959), so the overall franking of the letter mounted to 1.12 Dong. That was just a tad higher than the required tariff of 1.10 Dong for a registered letter to West Germany.
Mixed franking on a letter sent by a member of the North Vietnamese Armed Forces to a fellow Vietnamese in Hungary (note the mute cancel that was used) . The letter carried the red 12xu Five Year Plan stamp plus a 6xu Day of Labor stamp for a total postage of 18xu. Letters to fellow socialist countries only cost 12xu so this letter must have ben of the second weight level.
Mixed franking of the 12xu Aircraft Shot Down stamp together with a pair of the 6xu Labor Day stamp and 50xu Geneva Convention stamp for an overall postage of 74xu on an air mail letter to Paris, France.
Registration Nr. 101320