The caliber (if not altered) is 7.65x53, sometimes also called 7.65x54 or 7.65 Argentine.
The receiver was ground in Argentina to remove the National Seal before the guns were sold, then a requirement of Argentine law. Later, gun importers, prodded by collectors, impressed on the Argentine military that a better price could be had if the seal was intact, so the law was changed. As a result, surplus Argentine arms will be seen with both intact and ground seals. Those rifles were made of the best materials known at the time by some of the best craftsmen in the world. Many of those rifles imported were brand new and are simply beautiful.