Since 2000 Chech Republic is divided into 13 Regions called "Kraje" and 1 Capital City or "Hlavni Mesto" in czech language. Each Region is divided into Districts called "Okresy". The total of these Districts is 86.The Regions have been changed in the time and today some Districts belong to other Regions. We will use the old system according with the rules of the Czech Awards and Contest. So the old Regions were just 8: Prague City (A), Central Bohemia (B), South Bohemia (C), West Bohemia (D), North Bohemia (E), East Bohemia (F), South Moravia (G) and North Moravia (H). The Capital City Prague is also divided now into 22 Districts. For same reason for our purposes we will use the old 10 Districts of Prague. In this way the Districts 4th contains also the 11th and 12th; the 5th contains the 13th, the 16th and the 17th; and so on. The table contains all info about.

Each District is identified by a three-letter-code. The first figure is representing the Region and the other two figures are the code of the District. As example HOL means Olomouc (H= North Moravia, OL=Olomouc).

Even if the Regions are now more, and some Districts changed Region, their number is always the same: 86, and their codes did not change. So as example the District of Semily, code FSE, is now part of Liberec Region, that comes from the splitting of North Bohemia Region (E) into two new Regions, Usti Nad Labem Region and Liberec Region , but Semily's code still mantains "F" as first letter instead of "E".

By the way the new Regions names are: Prague (A), Central Bohemia (B districts), South Bohemia (C dictricts except CPE), Karlovy Vary (Districts DCH, DSO, DKV), Plzen (other D districts), Liberec (districts ECL, ELI, EIA and FSE), Usti Nad Labem (other E districts), Hradec Kralove (dictricts FTR, FJI, FHK, FNA and FRK), Pardubice (districts FPA, FCR, FUO and FSV), Vysocina (districts CPE, FHB, GJI, GTR and GZS), South Moravia (districts GBM, GBV, GBL, GVY, GZN, GBR and GHO), Zlin (districts GUH, GKR, GZL, HVS), Olomouc (districts GPR, HOL, HPR, HSU and HJE), Moravia-Silesia (districts HOS, HOP, HKA, HBR, HNJ and HFM).

This eQSL shows Kraslice as QTH. Also the info about the District is shown: DSO

Anyway if you put the QTH into the search engine of Wikipedia you will find this:

The District is Sokolov so the wanted code is CZ-DSO