This very topic was discussed in the most recent Mannlicher Collector's Association newsletter (#114). According to an excerpt from African Dangerous Game Cartridges by Pieure van der Walt:

"This is an Austrian cartridge and the Austrians had the silly habit of measuring case length from ahead of the rim, whereas the Germans measured the entire case length. As a consequence many Austrian cartridges are also known by their German designation which denotes the case as being 1mm longer. There is no dimensional difference."

My question for the more experienced out there: While the .375 H&H magnum is often used for brown bear, would the .375 Rimless Nitro Express be appropriate?

How does the round compare to the .30-06?