North Vietnam, National Sports Games, March 25th. 1966, Michel Nr. 438-440; Three stamps were issued. All in a nominal of 12 xu and in perforation 11.0.
Here is a letter that features the entire set twice 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. 101290