This construction method was used for the first, second, third and fourth states of the slipcase.

In use 1957 to 1960.

Image shows a fourth state slipcase, but the earlier slipcases were similar.

The sides and rear of the slipcase are formed from one piece of board, and the top and base from two more.  These are held together by the covering material - paper or Linson.

This type of construction is susceptible to splitting along the joins on the top and bottom edges, especially in the paper covered variants.