North Vietnam, 35 Years Young Workers Union, March 26th, 1966m, Michel Nr. 441; A single stamp with a 12 xu nominal was issued in perforation 11.0.
Very rare single franking of the 12xu value on a local letter sent to a member of the North Vietnamese Military (note the Hom Thu number in the address). Most local letters were destroyed in the multi-decadal war, the tropical climate or recycling due to raw material shortages.
Here is a letter that features two of the stamps and that shows an interesting mixed currency franking of old and new Dong. 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.
Registration Nr. 101300