This is the last chromatic harmonica the Koch company produced before they were bought by Hohner at the end of 1928.
I first saw this harmonica in the 1927 Koch catalogue:
What is special about this harmonica is the mouthpiece and slider.
The mouthpiece was made out of aluminum and had round holes. All other chromatic harmonicas at that time had brass mouthpieces with square holes. This mouthpiece was also thicker and connects without gaps to the cover plates. Modern chromatic harmonicas like the Hohner “Meisterklasse” and the Suzuki chromatics use a similar mouthpiece shape. Then there was the slider mechanism using an internal compression spring. It was very unique for that time. After buying Koch Hohner never used that slider innovation in their own chromatic harmonicas (only in some prototypes) until 1990 when the CX12 came out. As you can see, the slider uses a compression and not an internal torsion spring which was invented by Hohner in 1930 (more to that in my upcoming post) and still used in most of today’s chromatic harmonicas.
Update: apparently, Koch came also out with another version of the instrument called “Artist-Concert”.
It is not a chromatic harmonica. By pressing the button you can switch from C-Major to A-minor (harmonic minor).