It seems that as an afterthought, a commemorative stamp and souvenir sheet were issued on 20 May 1958 honoring “The 2nd Postal Week”, with no 1st Postal Week stamp issue in sight. The following year, 1959, in celebration of the 3rd Postal Week a new philatelic class of item was generated (on 20 May 1959); what we refer to as a “Special Sheet”.
The year 1960 saw the second Special Sheet generated for the 4th Postal Week, on 7 October 1960. Obviously, the Bureau of Posts planned to issue the third Special Sheet in celebration of the 5th Postal Week which was planned for 20 May 1961. This was curtailed due to Park Chon-hee’s Military Revolution, which began on 16 May 1961 (also referred to as “5.16” in Korea), 4 days before the planned release. Five years later, the completed Special Sheet was taken off-the-shelf and over-printed with the 6th Postal Week, and issued on 13 June 1966.
All scans are from Robert Finder‘s collection. For more information on the 1966 sheet see Robert’s article, the same for his article on the 1957 Boy Scout and 1958 Second Postal Week Issues.