[gyvel]

I could use a few more pictures, but I am thinking you have a sporterized Prussian 1866 Chassepot cartridge conversion. After the Franco-Prussian War, the Germans converted large numbers of captured French Chassepot 11mm needle fire guns to 11mm Mauser. Modifications were done to the bolt to allow the firing of centerfire cartridges and to make it cock on opening.

I had one such conversion of a cavalry carbine, and it sure looks an awful lot like it.

What bothers me, though is that the tang is much longer than that of a Chassepot.