Poland is divided into 16 Provinces (Wojewodztwa), each of them is divided into many  Districts, called in polish language Powiaty. The total of these subdivisions is 379. Usually the QSL cards report the Province (1-letter code) and the District (2-letters code). Sometimes they merge the codes getting a 3-letters code (e.g. R for Masovia and WM for Warsaw City give RMW or R-MW).

These are the 1-letter codes: zone 1: Z = Zachodnio-Pomorskie (West Pomerania); zone 2: F = Pomorskie (Pomerania) and P = Kujawsko-Pomorskie (Cuyavia-Pomerania); zone 3: B = Lubuskie and W = Wielkopolskie (Greater Poland); zone 4: J = Warminsko-Mazurskie (Warmia-Masuria) and O = Podlaskie; zone 5: R = Mazowieckie (Masovia); zone 6: D = Dolnoslaskie (Lower Silesia) and U = Opolske (Opole); zone 7: C = Lodzkie (Lodz) and S = Swietokrzyskie (Saint-Cross); zone 8: L = Lubelskie (Lublin) and K = Podkarpackie (Subcarpathia); zone 9: G = Slaskie (Silesia) and M = Malopolskie (Lesser Poland).  

But of course maybe the QTH is the only info on the QSL card. In this case the only help is Wikipedia.

This eQSL shows the QTH: Lwowek Slaski.

Put this data in the search engine of Wikipedia and the result will be:

The District is actually Lwowek Slaski, and the code is PL-LF