It is probably PART of the caliber marking. Norma (Swedish cartridge maker) created several Magnum cartridges under their name. They included the .308 Norma Magnum and the .358 Norma Magnum. They were based on the the .300 Holland & Holland Magnum case.
For more than that, we need more from YOU- like all the markings on the rifle, and pictures.
And DUDE- use the spellchecker!