North Vietnam, Union Conference Prague, June 26th, 1958, Michel Nr. D25; This official stamp was printed by the National Bank Printing Works in sheets of 100 stamps and in perforation 12.5.
Here is the mint stamp.
Most used stamps are cancelled to order with the typical Hanoi canceler that covers four stamps at a time like this one.
Postally used stamps are much rarer as it was prohibited to take stamps from Government offices. So most postally used stamps come from letters that were directed abroad. Here is such a specimen.
Registration Nr. 100106