A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... Gosudarstvennoi Bezopastnosti[?] (NKVD, 1934-1943) Ministerstvo Gosudarstvennoi Bezopastnosti[?] (MGB, 1946-1954) Komitet Gosudarstvennoi Bezopasnosti (KGB; ...