The Böhm harmonica company founded in 1850 and located in Klingenthal/Germany was one of the major harmonica manufactures in the first half of the twenties century.
The company produced quite an amount of innovative harmonicas. One innovation is described in the patent #DE470354 by Otto Hermann Böhm granted in 1929.
Otto Hermann Böhm
Pat Missin has a detailed technical explanation including pictures showing how this invention works. He also mentions that a similar idea was already patented (# US443607A) in 1890 by John Oefinger and Benjamin F. Butler of Greenfield, Massachusetts.
Chromatic Harmonica History 