“The Chromatic Harmonica” by Hohner

The next leap in chromatic harmonica development came from Hohner in 1910.

The Hohner chromatic harmonica was the first one with an external spring.
From that moment on the external spring was the de facto standard. It was used for practically all chromatic harmonicas until about 1930 with few exceptions.

Side note: the US patent# 1671309 for the Hohner Chromonica was issues in 1928. The application for this patent was made in 1926, some sixteen years after the Chromonica was first advertised.