Stanisław Prauss was indeed an experienced pre-war engineer. Another was Wsiewołod 'Jaki' Jakimiuk, who designed PZL P-11, DHC Chipmunk and led projects of Beaver and Sea Otter. Later in France - Barouder, UK - Sea Venom, and I think he ended his career as a consultant for Tornado.
Here is a well known picture of Szlenkier.
https://www.rp.pl/apps/pbcsi.dll/sto...-308049989.jpg