Illustrated below are two copies of DPRK 1356 (Scott’s), one with the 15ch denomination (right), but the other without any indication of denomination (left). I have no recollection about how these came into my hands. If anyone has any additional information about this apparent error, I’d appreciate hearing from them.

(Originally from KP February 2007, Vol. 52, No. 1)
Postscript 2018
The (North) ” Korean Stamp Catalogue 1946-2015″ lists this stamp as nr. 1394, but like the Scott catalogue there is no mention of a missing value error.