The S2-200 was developed by Louis Stange at the Rheinmetall company in Germany in the late 1920s. Because Germany was not allowed to be doing this sort of arms development at the time, Rheinmetall bought a controlling stake in the Swiss firm Solothurn AG, to make the product deniably Swiss. The gun itself is recoil operated, with a rotating locking collar connecting the bolt and barrel, rather like the Hotchkiss Portative. It was a design that had some early influence on the German MG34, although the German military declined to adopt it. Instead, it was taken into service in 8x56mm by both Austria (as the MG30) and Hungary (as the 31M). A third purchaser was El Salvadore, which purchased 47 examples in 7x57mm caliber.

After the anschluss in 1938, the Austrian guns were integrated into the Wehrmacht, where they were primarily used by mountain troops. Hungary did later make a version in 7.92x57mm, designated the 43M.

Many thanks to the Royal Armouries for allowing me to film and disassemble these two very rare LMGs! The NFC collection there - perhaps the best military small arms collection in Western Europe - is available by appointment to researchers:
https://royalarmouries.org/research/national-firearms-centre/

You can browse the various Armouries collections online here:
https://royalarmouries.org/collection/