The stamp reads Thonet Austria
And the paper label says Thonet Vienna
One of our readers asks if there are more details known of this Thonet Austria bentwood chair that belonged to her grandmother who moved from Germany to the USA in the 20ies.
Update Februaryu 2017:
Via 1stdibs I came across this chair:
The mystery seems solved it is seems alike the Thonet no 28. The back is a variation though.