Please Note: in the above picture on the right side: 375 Blaser Magnum (Left) vs. 30-30 Winchester (Right) for size comparison.

One Cartridge not a Box: 375 Blaser Magnum by norma, using older "NEW' ammunition. Factory loaded using (then) new norma brass cases with a "norma 375 Blaser Mag" Head-Stamp and loaded with a 300 grain norma ORYX or JSP (Jacketed Soft Point) projectile.    

History of the 375 Blaser cartridges:
The .375 Blaser Magnum was designed by Blaser Hunting Weapons and Norma Ammunition and introduced in 2010.
The cartridge develops a chamber pressure of  almost 70,000  foot pounds per square inch, pushing  a 300 grain projectile out the barrel at close to 2,700 feet per second with 4,800 foot pounds per square inch, more then enough for hunting dangerous game.
Norma is the only producer of factory ammunition in .375 Blaser Magnum as far as I know.(?)

It is about the same size as a 375 Daykota cartridge, however, over a 1/2" longer.

Very limited ammunition on this caliber in the USA