Sounds like your 1891 Argentine Mauser has been sporterized. It is possible but not likely that it has been rebarreled. They are not the srongest rifles, so they were often avoided for rebarreling projects. If it has the original sights, it is probably chambered for the original 7.65X53mm. Ammo is available, but you really gotta want to shoot it. Many do shoot them, other folks just park them in a closet.
I understand its been called the last antique rifle/ the first modern rifle.
I have heard it called the first modern cartridge in the last antique rifle, but neither is exactly accurate. The 1891 was a rework of the 1888 Commission Rifle design, so even though it was stronger it was still not very strong. 7.65X53mm was one of the first truly modern cartridges in design, even so the ballistics left something to be desired until the 1909 Argentine and its more powerful loading came along.