The difference is the color. If you look at the first group of 1/2d to 2-1/2d values, the first printings (SG 462-466 Scott 235-239) are all a deep shade, and the later printings (SG 485-489 Scott 258-262) are all a duller, paler version.
It is pretty easy to identify them once you have seen the difference between the stamps.
You can also look at used stamps with postmarks. The deep color issues were printed beginning in 1937. The paler shades were printed beginning in 1941. The Commonwealth catalogue indicates the color change was a wartime economy measure. (Probably to conserve ink supplies.)