Definitely not Austria.
I would say to not place it under the Duchy of Teschen. That is under Bohemia, and... I do not think that is right for this note.
While Cieszyn was split between Poland and Czechoslovakia, the there were two different councils. Poland had the 'Rada Narodowa Księstwa Cieszyńskiego' and Czechoslovakia had the 'Národní výbor pro Slezsko', according to this:
https://en.wikipedia.org/wiki/Cieszyn_Silesia
I can see this on the note:
But I cannot seem to find reference to the Czech council. Perhaps I am missing something (I cannot read the note), but if I am not, I'd say to place this in a Polish Notgeld section.
(We will need a Polish Notgeld section for coins too, so... it would be nice if this was
created.

)