A fairly rare and quite valuable 1930s commercial predecessor to the Kar98K Mauser of WW2 fame. The Mauser Standardmodell was a "commercial and sporting" rifle produced for export, for "sport" and as an excuse to start producing military rifles in interwar Germany. Many were sold to China, many were sold domestically in Germany, and they are a highly prized collectable Mauser in the US. Value should range between $800 and $2,000 US if the bore is in decent condition, though the replacement of the rear sight with a Marbles hurts value slightly.
The ammunition is actually marked quite clearly 8mm Mauser on the box, and before anybody confuses you, this should be a standard commercially available 8mm Mauser, not any of the wildcats or obsolete European loadings.
Though, for sake of safety, it would not be inadvisable to carefully check the depth of the chamber- some of these rifles were modified to use the "8mm-06" cartridge derived from a US .30-06 casing loaded with an 8mm bullet, as .30-06 ammunition was cheap to purchase in the 1950s and easily fitted with a new 8mm bullet to use the rifling of a converted Mauser. A converted rifle will have a 63mm deep chamber and swallow a .30-06 completely, an unconverted 57mm chamber will only take 3/4 or so of a .30-06 casing.
Congratulations, it's an enviably cherry example of an uncommon rifle.
Also, it's generally referred to as the Magazine Floorplate or triggerguard assembly.
