Dean,
Hope this helps.Frantisek Dusek established a gunsmithing shop in 1908 in the town of Opocno where he marketed Rifles, shotguns and gunsmithing supplies. He did not get into the pistol manufacturing business on his own til about 1937 when he introduced a Browning type pistol (.25) without a grip safety. These were marketed with the name Singer on the grips. (The same name as the Spanish pistols he was importing and marketing previously before his manufacturing enterprise.) These pistols also had "Duo" on the slide, which I'm sure you know is a contraction of Dusek,Opocno.The higher cost of Dusek's pistols and superior quality compared with the older Spanish ones undoubtedly caused the name to be changed to DUO. Dusek refused to cooperate with the German Occupation forces, so the factory operated without him. Some of these pistols are also marked JAGA as well as DUO. Your pistol, manufactured in "44" was manufactured by the Dusek Co. under German supervision. Very possibly the town name "Opocno" is spelled the German way "Opotschno". Further information is available in "Know your Czechoslovakian Pistols" by R.J. Berger.
Dave