I've never seen one of these before. Guess it could be a very small assault gun. I'm talking a small "assault gun", not a "small assault" gun if that helps or makes any sense. Anyway, time for the peanut gallery to get into action.
15 Cm Sig 33 on a PzKw I chassis, an early and obviously somewhat crude German SP. These were used in the early campaigns of WWII. There's a well-known film clip on one engaging some French holdouts in a house in 1940.
Also, there's video in "The World At War" of an SP assaulting the main post office in Warsaw. Is this the same type?
I don't know about Poland, but here is the thing in action in France. This is the clip I referred to.