send the seller a pm and asking for pics showing the bolt head. I am 100% sure he dont realy know what for a chamber his rifle have. there were polish mosin rifles rechambered into 8x57IS between the wars so its possible and could be. maybe such a rifle ends civilian hand in austria after ww2. its a typical ferlach gunmaker build Stutzen and its not realy possible to say if it was made after 1918 or 1945. Austria rechamber Mosin rifles to 8x50R Mannlicher in the great war. could be its such an old warhorse.

so we have this possibilty's: 7,62x54R
8x50RMannlicher
8x57IS
just another sporting round

anyway, its a lovely stutzen and I dare to say it will sell for low money because the stupid dont appreciate it aynmore.
dont fit into my collection and gun safe room is scarce