With the Leica II from 1932 and the lenses that were available at the time, the system hardware for Oskar Barnack's small-format was, in fact, established.
With the Leica III (known in the USA as the “Model F”, not to be mistaken with the later Leica IIIf) a new evolutionary phase began. The main difference between the III and the II was the possibility of longer exposures (1, 1/2, 1/4, 1/8 and 1/20 s), which, considering the low light films of the time (approx. ISO 12 – 16), was a welcome innovation.
This was visible externally in the form of a separate front dial for the long exposures located on the right next to the lens. And there was something else that was new with the Leica III: strap lugs were now an integral part of the camera body. The previous models had to be carried either without a shoulder strap or in a frequently cumbersome carrying case – a real innovation in its day.