A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... (GPU, 1922-1923) Obyedinennoye Gosudarstvennoye Politicheskoye Upravleniye[?] (OGPU, 1923-1934) Narodnyi Kommissariat Gosudarstvennoi Bezopastnosti[?] (NKVD, ...