Looks like armored trains and cars badge / patch - shape of the helmet is the same. It is probably privates' and NCOs' patch. The badge / patch was used until 1926. It is very hard to find photos of the same patch as shown here. Please find some links for reference with visualisation:
zaloga pociagu pancernego "Gromoboj" - Ogólnopolski miesi (here is metal plate),
I cannot recall any of the articles that could refer to the issue. There is no suitable internet source as well. The only compendium known to me with very poor quality drawing of the patch is Henryk Wielecki book - Polski mundur wojskowy 1918-1939.