Stamp booklets were first put on sale on 21 August 1931. Booklet construction was a lightweight cardboard cover stitched on the left-hand side with panes of stamps (usually four panes), advertising panes and interleaving bound in. Until 1956 most booklets had half of the front cover devoted to advertising, and until 1963 booklets also had serial numbers on the front cover: two numbers indicated the year of issue and serial number (in that order on the early issues but with the order reversed for later issues), until 1963 when the serial numbers ceased.