Geverson, your rifle is a Commercial Mauser, Oberndorf, sporting rifle type B. Accordingt the 77765 serial number it was made in 1913. Later, likely in Brazil, it was rebarreled with a Mauser Military rifle barrel, probably in 7x57, the former Brazilian military cartridge. Over the years Brazil bought many miltary Mauser rifle models in 7x57 to equip it's army and various police forces. The original Mauser stock, action and bottom metal were retained. The Mauser rear sight sleeve was attached to the "new" barrel. The front sight and it's base are not original, as is the crudely fitted rubber recoil pad. The front sling swivel is a replacement too, simply screwed into the foreend.The stepped barrel is typical for most military Mauser rifles, from 1898 on. The steps allowed the military barrels to expand lengthwise from heating up, without altering wood to metal pressure.