Tribune Stamp Co., a subsidiary of Herrick Stamp Co., issues a "Wholesale New Issues Stamp Bulletin", every two weeks. One of the many countries that they list is North Korea. LINN's Stamp News, publishes weekly, a section entitled New Issues (stamps of the world - alternating between five areas of the world).
In the table below "TRB#"=Tribune Stamp Co. Bulletin No.
This column may also contain the date that Herrick's web-site updates their North Korea stamps for sale.
I have identified the LINN's edition, in which New Issues - Asia described the North Korea stamp issue.