A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... Socialist Republics (USSR) Cheka (1917-1922) Gosudarstvennoye Politicseskoye Upravleniye[?] (GPU, 1922-1923) Obyedinennoye Gosudarstvennoye Politichesko ...