Kontakt 2 / Apple EXS24 / Digidesign Structure
The button accordion was multi sampled approximately in minor 3rds. In the above photo you see the 2 rows of the melodic buttons (21 of them) and on the bottom you see a set of buttons which create octaves as well as other intervals. Two different notes are produced for each button depending on whether you are moving the bellows in or out (much like a harmonica).
These are 16 bit stereo samples (dithered from 24 bit) and the entire sample set is about 25 megabytes.