The pic of the Rigby was posted just to show the action I do not know if it was re-barreled. Looks original to me from other pics of the rifle.
I read that Rigby commissioned the magnum for the .350 Rigby and .416. I believe they had exclusive rights in England for some time.The .350 was introduced in 1908 a few years before the .416 (1911).
Magnum Mausers were made until WWII started (let's say 1939?) so that would be a 31yr run. Mauser offered the 10.75x73 (.404NE)with the magnum action as did Abbeser and Merkel and I am sure other contemporary rifle builders also had access to this such as Schuler and Gibbs(.505,1911).
After the war the French confiscated Mauser machinery and produced the Brevex 300 and 400 magnum Mauser action.