From a Polish site, http://www.wojsko18-39.internetdsl.pl/wrzesien/mozdzierz120.htm. It's designation is given as the wz.40. Since the Poles usually use the year a weapon was adopted by the military in their designations, this set off alarm bells. So I ran the page through a translator. Apparently the gun is one of two prototypes made, and it's unclear to me if either saw action in 1939. The gun had a threaded barrel, and did quite well in its field trials. Production was scheduled to start in...1940. I checked a few other on-line resources, and this is the only 120mm mortar mentioned.

Polish 120mm Mortar

Prototype of a polish tank destroyer. It was armed with the 37mm Bofors gun. They were used during the September Campaign by the 10th Motorized Cavalry Brigade. 2 prototypes were built. 

TKS-D

Abandoned Artillery tractor C2P with 75 mm gun Schneider Mle 1897