yes ,this is the big unknown, its a Haenel Model 1900

Haenel Model 1900 and Model 1909 were ubgrades by C.G.Haenel in Suhl of the M 88 action the same like the Mannlicher Schönauer Model 1900 was an ubgrade from the M 88 ( what became later the Model 1903 Mannlicher Schönauer). The reason was the Mauser Model 1898 what makes all other rifles looking very old then.
Kuduae had wrote a little bit about it here

https://www.germanhuntingguns.com/archives/haenel-c-g-of-suhl/

and we have them from time to time in this forum

http://forums.nitroexpress.com/printthread.php?Board=mannlicher&main=319322&type=post

http://forums.nitroexpress.com/showflat.php?Cat=05826500&Number=318820&an=0&page=8

Please don't get me wrong but yours looking like a basic version, maybe sporterised by Haenel by using rifles made for military contracts to china or south america. Caliber must be the beloved 7x57 mauser.