The symbol named is “Kościesza”, this is old polish coat of arms used from XIV century.
In years 1925-1939 was used as coin-sign by Mennica Państwowa (State Mint)
In Poland only one produced coins from brass in year 1923, this was:
Theoretically using for this production (weight of coin*quantity) about 131 tons of brass.
Fact, this was very poor qualitatively coins, next series these coins was produced from bronze or coppers. For production this coins was used brass with composition: Cu 60%; Zn 39,5%; Sn 0,5%
this is little different from brass for ammo case.
In my theory:
In the latter part of 30’s (1937-38) was tested possibility some factories of change from normal production to production of war.
Mint probably was not only tradesman of brass but produced also discs of brass for cases as semimanufactured article, because first stage of production of case is very similar to coins.
(Picture is from mint from that of period - by NAC)
Mint was located near Zakłady Amunicyjne “Pocisk” (symbol “Pk” on case) because as producer case/ammo is this factories.
If goes for marking given by AveDanzig , situation can be similar.
Producer of case/ammo is Zakłady Amunicyjne in Skarżysko. We should seek smelting works situated near it .But in this instance would be probably only tradesman of brass.
Unfortunately I cannot find any informations which would confirm this theory.