Sorry if this link is already known here. I thought it was interesting to post.
From "weltsauerstoff" youtube channels, I found this page http://dicta.bplaced.net/konzertina/ where detailed musical informations are given in order to play a 20 buttons concertina (he has a Silvetta).
German speaking page, but some on-line translation can help !
I was personnaly impressed by the harmonic style.
Have a good day,