All of the stamps of North Korea issued before 1950, whether reading from the right or from the left, are inscribed “postage stamp,” yu-pyo (우표). In 1950, however, some came to be inscribed “postage,” yu-pyon (우편). This is the case with all of the stamps in the 5th Anniversary of Liberation series, the Liberation of Seoul issue, and one of the stamps in the medal issue, the 1 won red-orange lithographed. In 1951, a few stamps, the Ri Sun Sin stamp and the six stamps in the 6th Anniversary of Liberation set, are similarly inscribed. No later stamps seem to bear this inscription. All this is obvious from the catalogues and is of minor interest.
The interesting part comes from looking at the franks on postal stationery–catalogue numbers below are from the Pyongyang KSC postal stationery (1999) and stamp (2007) catalogues. Postage (yu-pyon) would seem to be the logical expression here, and, indeed, it seems that all cards and covers until about 1960 are so inscribed, with a few exceptions. A singular case is the 1955 cover for the 10th anniversary of liberation, No. 3 in the KSC catalogue, which bears a frank based on No. 68 in the stamp catalogue, reading yu-pyo. After about 1960 almost all franks read yu-pyo.
The most interesting cases are those before about 1960 for which dual franks occur. We note five examples.