From: Greg's & Blaise's Bandoneon and Tango Pages (archived at archive.org): "When it comes to defining what constitutes a bandoneon, there is the possibility of some confusion. The bandoneon used for Tango will invariably have two reeds per note; one will be at normal pitch, the other will be one octave higher. These reeds are always tuned to a pure octave; no tremolo should be audible when a single note is played! Another important attribute of instruments destined for the South American market is the configuration of the keyboards. The instrument originally had 60 notes disposed over 30 buttons, but that quickly changed to 130 notes. By the beginning of the 20th century, a 142-note version with 71 buttons had become standard in South America."
The bandoneon was developed throughout Germany under various sizes and systems. One of these many different models,the "Reinlander"(from the Rein district) was exported to Argentina at the very end of last century,whereas the "Chemnitzer"(from the city of Chemnitz) was brought to USA by Polish and Czech migrants. Germany used to sell a lot of musical instruments to both north and south America,i.e. the harmonicas used in blues, the melodeons used in Cajun music and all kinds of accordions in the brazilan Nordeste, Colombia and so on.
Bandoneon was very quickly adopted in Buenos Aires and became the symbol of Tango. It was never built there.Most bandoneons were made by the german maker ALFRED ARNOLD from 1911 untill few years after the war.
The argentinian bandoneon is a two voice instrument (each note being doubled at the superior octave) with 71 buttons. Each button plays a different note depending on whether the bellows is opened or closed (unproperly called diatonic). Around 1925, Charles Peguri,an Italian accordion player and repairer settled in Paris designed a new keyboard where each button produced the same note regardless of whether the bellows was open or closed (called "chromatic") which has been widely used in France For playing Tango, the bandoneon must be tuned without vibrato (the two voices being precisely an octave apart).