To get an opinion as to how the Polish partisans would have acquired the M31 Suomi submachineguns. (Finnish designation: Kp/31), I posted photos of the AK unit operating in Radom-Kielce in another forum.
In response it was suggested that those SMGs would have been captured from German Police irregulars or SS Police Division units and possibly Waffen SS. Here is a photo showing a Kp/31 Suomi being used by member of the bodyguard of SS and Police Leader of Warsaw, Jürgen Stroop, during the liquidation of the Warsaw Ghetto in 1943. (left background)
Also a German SS soldier is armed with a MP28 Bergmann submachinegun on the right. One of the previous AK unit photos shows a partisan possibly armed with a Bergmann.