The 375 Holland & Holland Magnum was developed in 1912 and was one of the earlier belted-magnum cartridges on the market. Designed to use the new smokeless propellant cordite, the 375 H&H Magnum quickly became popular for hunting both medium, large, and dangerous game especially in Africa.
Read more about History of Hornady 375 H&H Magnum Ammo here: https://ammo.com/brands/hornady-ammo/hornady-375-h-h-ammo#caliber-history