During the 1980’s some manufacturers developed free bass only accordions but the feeling of the bass was not the same as the treble, as the bass note-board was not intended for single-note playing with the thumb being designed for standard bass. The accordionists stayed with the converter model.

Fisitalia writes: We have developed a completely new “Bayan-Ergo” free bass accordion design with many advantages:

– The clearly slanted bass side and larger bass buttons (same as treble) allow the player to have a more natural hand and finger position, facilitating easy use of the thumb;
– The thumb can also be used, the same for the left hand as for the right hand;
– Easier and freer hand movement and glissandi are possible;
– The legato performance is significantly easier;
– Greater span of the left hand using the thumb;
– With the thumb the player has increased reach;
– Five-note chords in the left hand are now possible;
– Lowest tone in the bass: Contra-C (in accordance with an organ or a harmonium).
– Composers may use the thumb of the left hand for their works;”

Full Information with extra pictures: Ergo_English and Ergo_Deutsch