Following the "Specialized catalog of Polish coins of the 20th and 21st centuries". by Mr. Jerzy Chalupski, we come across the following: "August 26, 1939. The President of the Republic of Poland signed a decree published on the same day introducing steel coins with denominations of 20 and 50 groszy into circulation. Steel fifty-groszy coins were to be introduced into circulation and function in parallel with nickel coins."
The coin has so far been erroneously attributed to the General Government, but was in fact already minted during the Second Republic.