Wow, whoever owned it before has gone to the trouble of milling out a slot for the new sight. As mentioned earlier, Predom Lucznik 188. Not worth a fortune... even in good nick. I have one of these and the Predom pistol as well. In good condition, no more than 50-60 quid I would guess.
CZ 200T/200 S Hunter/200 S Green/Slavia ZVP/612/614/615/618/620/622/624/625/627/630/631/634/ZVS Perun 734/CZ236-1/Tex3/86/Brno ZII/ZV3/ZV4/VZ47/APP661/Relum/Mars 618/Stella 518/Predom 188/170/Yunker 1/2/Lov21/Baikal IJ22/IJ38/Vostok/IZH61/IZH53M/MP512M/MP654K/MP661K