This CZ Model S rifle is one of many prototypes made between the world wars in Czechoslovakia in an effort to develop a military semiautomatic rifle. Similar efforts were underway in most other countries at the same time (basically every place that had a mature arms industry), and a huge variety of ideas were tried out.
In this case, we have a gas operated, tilting bolt rifle design. It is chambered for the 8×57 Mauser cartridge, and uses a 10-round detachable magazine. This rifle has a 1929 acceptance mark, which dates it nicely for us. It was designed by a team including one of the Holek brothers, and did better than its competition in trials – but not well enough to be accepted by the Czech military or to attract any outside commercial interest. Holek would go on to design the ZH-29 rifle, and CZ would move on to a series of other designs that ultimately led to naught.