I recently acquired a few pre-war 10 hole chromonicas and a couple Koch 10 holers. These are very interesting harps as they tuned in the style of a standard diatonic Richter, as opposed to the solo tuning which is common to chromatic harmonicas. Essentially, the harps are two complete diatonics that are 1/2 step apart and the player can switch between notes using the button slide. The purests call them a slide-diatonic although I still consider them chromatic and evident