František Dušek, born in 1876, established himself as a gunsmith in the town of Opocno in 1908. He was anti Nazi which lead to his factory being seized in WW2. The duo is a small pocket pistol based or inspired by the browning designs including the baby browning and FN. CZ is a eastern European company which has been operating since the 1930’s.
The time Line - The Duo was manufactured starting off life as pistols produced as Slavia pistol (1920) in the Bohemian town of Kdyne, whick became Posumavska Zbrojovka Kohout which then became the Mars Pistol to finally become the DUO. This pistols was then refinished and stamped.
This DUO as it is marked 1949. The duo Grips strongly suggest the pistol was pre 1948 as the grips do not bear the Z symbol which occurred after Nationalization of the soviet republic. The grip with DUO being post 1938.
The references suggest supplies of “blanks” of the slavia would have been exhausted and Spanish conflicts prevented import from Spain of a similar gun.
The conclusion then is that this pistol is the refurbished MARS Pistol.