North Vietnam, “Steel-Island” Con Doa, June 1st, 1966, Michel Nr. 444; Con Doa (sometimes also called Con Son) was a prison island that was first built by the French colonist in 1861 to detain and demoralize the local population. After the French withdrew in 1954 they turned over the island to the South Vietnamese Army who, under the supervision of American advisors, used it to detain Viet Cong prisoners (http://www.globejotting.com/con-dao-part-one-the-prisons-of-con-son-island/). Obviously the prison was despised in North Vietnam and hence postal authorities issued a single 12 xu stamp that reminded the population of the horror of Con Doa. The stamp was perforated 11.0.
Here is the perforated stamp.
This stamp was not officially released imperforate however a very small number of imperforate trial proofs have come to market. Here is one of them.
Rare express mail letter with one of the 12 xu stamps sent to Klewitz in February of 1970. The letter is overall franked with 1.62 Dong which is a tad above the required standard letter postage for a registered express mail letter to West Germany (1.60 Dong).
Mixed franking on a letter sent to Budapest, Hungary including the 12 xu Steel Island stamp for an overall postage of 36xu. Since the standard letter rate to Hungary was only 12xu this is an indication that the letter was of the 3rd weight level.
Registration Nr. 101340