Well it would be in Special Events.
You have the invasion of Poland, Italian Greek War (CR.42s and vs PZL P.24), and then maybe some Eastern Front early stuff. I know Bulgaria never went to war versus Russia but not sure if the Romanian PZL P.24s were involved in the invasion of Russia or not.
Very early war stuff, as said, the British used a lot of Gladiators (Norway, Crete, Greece, Ethiopia, Iraq, etc.), Italians had CR.42s (Greece, North Africa, Ethiopia), PZL P.11/ PZL P.24 (Poland and Greece), I-15/I-153 (Russia). I think that rounds out the early war biplane and transitional fighters (high wing monoplanes) that were in use for 1939 - 1941.