Congratulations Kai-Petri!
It was Germany.
S. Suprun was a member of a Soviet aircraft purchase comission that visited Germany March 1941. There he was given a chance to fly captured Spitfire Mk IA and P-40.
The info is from
http://www.apali.fi/uk/uk.html click "aviation books", then "Red Stars".
Your turn, Kai- Petri
Regards, Juha